Title of Invention: Card Metaphor For Activities In A Computing Device Abstract: In various embodiments, a card metaphor is established, in which each activity can be represented within an area of the screen referred to as a card. In various embodiments, any of several display modes are available for viewing, interacting with, manipulating, initiating, and dismissing cards. A persistent positional relationship can be established among cards, represented by a one-dimensional sequence. Newly opened cards are generally placed at the end of the sequence, although a new card that bears a relationship to an already open card may, in some embodiments, be placed adjacent to the already open card. In various embodiments, cards may be grouped, with such groups being represented as stacks of cards or by other visually distinctive means. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims priority as a continuation-in-part of U.S. patent application Ser. No. 12/126,145, filed May 23, 2008, for “Navigating Among Activities in a Computing Device”, Atty. Docket No. PLM5729, the disclosure of which is incorporated herein by reference. The present application is related to U.S. patent application Ser. No. 12/115,992, filed May 6, 2008, for “Extended Touch-Sensitive Control Area for Electronic Device”, Atty Docket No. PLM5728, the disclosure of which is incorporated herein by reference. The present application is related to U.S. patent application Ser. No. 11/379,552, filed Apr. 20, 2006, for “Keypad and Sensor Combination to Provide Detection Region that Overlays Keys”, the disclosure of which is incorporated herein by reference. FIELD OF THE INVENTION In various embodiments, the present invention relates to application management in computing devices, and more particularly to systems and methods for implementing a card metaphor for activities such as software applications in such devices. DESCRIPTION OF THE RELATED ART Many currently available computing devices run operating systems that offer users the opportunity to run several applications at the same time, and even to perform several activities simultaneously, within a single application and/or among two or more applications. For example, a user may open an e-mail application, a word processing application, an on-screen calculator, and a web browser, so that all of the applications are open concurrently. The user may open several documents within the word processing application, and may open several e-mail messages. Such an arrangement allows the user to move from one task to another by keeping these various applications and activities open at the same time. Conventionally, such an operating paradigm is accomplished by the use of multiple overlapping windows within a graphical user interface. A desk-top metaphor is often employed, wherein the user can move and resize windows on a display screen by direct manipulation. Thus, each application may have a window or several windows; at any given time, one window has focus (usually indicated as such by being situated on top of the other windows). Other windows may be visible, invisible, or partially visible (for example if they are partially or completely obscured by other windows). A user can switch from one task to another by causing a different window to have focus; this is commonly accomplished by clicking on a visible portion of the window being activated, or by clicking on a button or icon representing the window or application in a task bar, quick-launch area, or similar construct. In some operating systems, pop-up menus and/or keyboard shortcuts may be available for switching among applications or documents. Overlapping windows are a highly effective mechanism for application management. They exploit the user's spatial memory by associating a particular area of the screen with a particular task being performed. A user may find it easier to return to a previously opened window when he or she remembers that the window was placed at a particular location on the screen, as compared with a list or menu of open windows that does not provide a model of spatial placement and stability. One issue with conventional window-based mechanisms for application management is that a large screen size is usually needed. Users faced with limited screen space must choose between, on the one hand, making windows smaller and thus reducing available workspace within each application, and, on the other hand, stacking windows atop each other so that only one window (or very few) is visible at a time. Reducing workspace can make it very difficult to use an application, particularly for applications such as word processing, photo editing, drawing, and the like. Stacking windows reduces the ease with which the user can switch from one application or document to another, as it is more difficult to see what is currently open and to find the appropriate window to be activated. When windows are stacked, users usually resort to navigation via taskbar, quick-launch area, menu, keyboard shortcut, or the like, thereby reducing the advantages of the overlapping window paradigm. Some systems, such as the Mac OS operating system offered by Apple, Inc. of Cupertino, Calif., offer a mechanism for navigating among open applications by temporarily moving and shrinking open windows, moving the windows apart from one another so that they do not overlap. The user can then activate one of the windows; the original window positions and sizes are then restored, with the activated window on top. This feature is effective in allowing users to quickly switch among open applications and documents, but still requires a relatively large amount of screen space so as to effectively show the temporarily shrunk windows when the user is to select among them. Mobile devices such as smartphones, personal digital assistants, music players, handheld computers, and the like are becoming increasingly powerful. Many are now able to run several applications concurrently, and to perform sophisticated functions that rival traditional computing devices. However, most such devices have extremely small screens, when compared to laptop or desktop computers. Thus, the conventional window-based techniques for navigating among open applications and documents do not function very well on mobile devices. There is insufficient screen space to display multiple overlapping windows. In fact, many such devices do not even have enough space to display a task bar or quick-launch area. Rather, the entire screen is often devoted to a single application, document, message, task, or activity, so as to make the most effective use of the extremely limited amount of space available. SUMMARY Related U.S. patent application Ser. No. 12/126,145, filed May 23, 2008, for “Navigating Among Activities in a Computing Device”, Atty. Docket No. PLM5729, the disclosure of which is incorporated herein, describes various embodiments for running two or more activities concurrently on a computing device, and provides at least two modes for interacting with the activities. The user may toggle between the modes as desired. A button or other control is provided for switching between full-screen mode and card mode. In various embodiments, a card metaphor is established, in which each activity can be represented within an area of the screen. For purposes of the description herein, these areas of the screen are referred to as “cards”. Although in some ways cards resemble windows as are known in the art of desktop user interfaces, in various embodiments cards can be manipulated in particular ways that are not generally available to conventional window-based graphical user interfaces; such manipulations are described in more detail herein. One skilled in the art will recognize that the term “card” is used herein merely as a convenient term for referring to an area of the screen that contains information related to or generated by an activity such as a software application. In various embodiments, any of several display modes are available for viewing, interacting with, manipulating, initiating, and dismissing cards. In one embodiment, in a full-screen mode, one activity occupies substantially an entire display screen. The card thus fills substantially the entire display screen, although in some embodiments some areas of the screen may be reserved for status indicators, alerts, messages, and the like. In one embodiment, a second mode is available, referred to as “card mode”. In card mode, one activity is visible within a card, and a portion of at least one other card is also visible. Thus, a card that has focus (i.e., that the user is interacting with) is visible in full, while at least one other card is only partially visible. In the card mode, the user can cause cards to move on the screen, so as to change focus from one card to another. For example, the card having focus can be moved off the screen, to be replaced by a new card that is then given focus. In one aspect, the cards are ordered in a sequence. In one aspect, the cards move in concert with one another; pushing one card off one edge of the screen causes a new card to appear at the opposite edge. A persistent positional relationship can be established among cards, based on the established sequence (although the user can rearrange the sequence if desired). In one embodiment, a third mode is available, referred to as “shuffle mode”. In shuffle mode, two or more cards are shown in full. The size of the cards may be reduced so that more cards can be shown simultaneously. In addition, portions of one or more other cards may be visible. For example, in one embodiment, three cards can be shown in full in a horizontal row, with portions of two other cards shown to the left and right of the fully displayed cards. In one embodiment, shuffle mode facilitates easier reordering and manipulation of cards, and is associated with certain behaviors (described below) to optimize such operations. In one embodiment, shuffle mode resembles a zoomed out view of the cards presented in card view. In one embodiment, the user can zoom out to various degrees, so that several different varieties of shuffle mode can be presented, for example with different numbers of cards on the screen. In one embodiment, when more cards are displayed, they are displayed at a smaller size. In one embodiment, cards are presented in a linear arrangement in shuffle mode, preserving the linear sequence used in card mode. In one embodiment, a fourth mode is available, referred to as “grid mode”. In grid mode, a larger number of cards are shown in full. In one embodiment, cards are scaled down in size so that all available cards are displayed simultaneously. In another embodiment, all cards are displayed to a predetermined maximum number of cards; if more than the predetermined maximum are available for display, scrolling is implemented to allow access to non-displayed cards. In one embodiment, cards are presented in a grid arrangement in grid mode. In one embodiment, a user can manipulate cards in different ways depending on which mode is currently active. In one embodiment, different interactions are available in different modes, and/or certain user actions can be interpreted differently depending on which mode is currently active. The particularities of such differences are described below for various embodiments of the invention. In one embodiment, cards are active, regardless of the current display mode, so that the information represented in a card is substantially current and is updated substantially in real-time. In one embodiment, the user can interact with cards regardless of display mode, so that user interface elements within cards are active regardless of the display mode. In another embodiment, in some display modes, some or all cards may not be active, so that the data shown therein can be “frozen” while in a particular mode; such an embodiment may be useful, for example, when available system resources preclude real-time updating of all cards without undesirably degrading system performance. Similarly, “frozen” cards may be temporarily inhibited from accepting user input in some embodiments. In one embodiment, a persistent positional relationship is established and maintained among cards. The positional relationship can be a one-dimensional sequence or some more complex two-dimensional relationship. The positional relationship is persistent in the sense that it does not change unless the user indicates that it should be changed, or some other event takes place that indicates a change is warranted (such as non-use of an application, or an application crashing or being dismissed). In one embodiment, cards are initially positioned according to the order in which the corresponding activities (such as applications) were launched. New cards can be launched, for example, when a new application or activity is launched, or in some cases for an application that is already running (for example if a new message is being composed in an email application). Thus, in an embodiment where a one-dimensional sequence of cards is implemented, a newly-activated card would be placed at the end of the sequence; for example, if a horizontal row is used, the newly-activated card could be placed to the right of the right-most card in the sequence. A user can, of course, reposition cards as desired. In one embodiment, if a user launches a new activity (such as an application), a card (or more than one card) for the activity is created, with a position at the end of the sequence. If the user attempts to launch an activity that is already running, in one embodiment the card is not moved within the sequence of cards, but the current view of the sequence is shifted so that the card for the activity is given focus and presented in a central location on the screen. Alternatively, in another embodiment, if the user attempts to launch an activity that is already running, the card for that activity is moved to the end of the sequence. As described above, in one embodiment, newly opened cards are initially positioned at the end of the sequence. In another embodiment, some newly opened cards may be placed at locations other than at the end of the sequence. For example, any new cards for an already-running activity are placed adjacent to previously existing card(s) for that application. These cards, referred to as “sibling cards”, are therefore positioned so that cards for a given activity are located adjacent to one another in the overall card sequence. For example, a new card for an email application may appear when a user initiates a reply to an email message; the new sibling card for the reply can be positioned adjacent to the existing card for the original message. In one embodiment, the user is free to move sibling cards so that they are no longer adjacent to one another; in another embodiment, sibling cards cannot be separated from one another, although they can be moved as a group. In one embodiment, a visual paradigm can be employed to indicate groups of cards. For example, in one embodiment, card groups can be denoted by stacks; all cards within a particular group are displayed in a manner that resembles a stack of cards. Alternatively, card groups can be indicated in other ways that distinguish them from non-grouped cards: for example, card groups can be shown via a distinctive color scheme, different sizes, different vertical or horizontal positioning, bracketing, highlighting, spacing, or any other mechanism. In various embodiments as described below, a user can interact with groups, separating their constituent cards as desired, creating new groups, changing the order of groups, and the like. In one embodiment, where stacking is used to indicate groups, cards can be “unstacked” at times, even while preserving their group relationship, either automatically or manually to facilitate certain types of interactions and viewing of constituent cards. While cards are unstacked, in some embodiments, an alternative mechanism, such as bracketing or distinctive coloring, is used to remind the user that the cards are still members of a group. The various features described above and herein can be implemented singly or in any combination, as will be apparent to one skilled in the art. Additional features and advantages will become apparent in the description provided below. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. One skilled in the art will recognize that the particular embodiments illustrated in the drawings are merely exemplary, and are not intended to limit the scope of the present invention. FIG. 1 depicts a display screen showing a launcher for initiating an activity such as an application. FIG. 2 depicts the display screen in full-screen mode, wherein an activity occupies substantially the entire display screen, according to one embodiment of the present invention. FIG. 3 depicts the display screen in card mode, including a card having focus and two partially displayed cards, according to one embodiment of the present invention. FIG. 4 depicts the display screen in card mode after the user has moved the cards, according to one embodiment of the present invention. FIG. 5 depicts the display screen in full-screen mode after the user has designated a new card as having focus, according to one embodiment of the present invention. FIGS. 6A through 6F depict a sequence for switching from one card to another according to one embodiment of the present invention. FIG. 7 is a conceptual depiction of a sequence for switching from one card to another according to one embodiment of the present invention. FIGS. 8A and 8B depict a sequence for dismissing a card according to one embodiment of the present invention. FIGS. 9A through 9D depict a sequence for rearranging cards according to one embodiment of the present invention. FIG. 10A depicts the display screen in card mode according to one embodiment of the present invention. FIG. 10B depicts the display screen in shuffle mode according to one embodiment of the present invention. FIG. 10C depicts dismissal of a card in shuffle mode according to one embodiment of the present invention. FIG. 10D depicts reordering of a card in shuffle mode according to one embodiment of the present invention. FIGS. 10E, F, and G depict various examples of grid mode according to embodiments of the present invention. FIG. 11A depicts the display screen in full-screen mode wherein four cards are open, one of which has focus, according to one embodiment of the present invention. FIG. 11B depicts introduction of a new card in full-screen mode, in response to launch of a new activity, according to one embodiment of the present invention. FIG. 11C depicts rearranging of a card sequence in full-screen mode, in response to launch of an activity that is already open, according to one embodiment of the present invention. FIG. 11D depicts a focus shift without card reordering, in full-screen mode, in response to launch of an activity that is already open, according to one embodiment of the present invention. FIG. 12A depicts the display screen in full-screen mode wherein seven cards are open, one of which has focus, according to one embodiment of the present invention. FIG. 12B depicts introduction of a sibling card for an open activity in full-screen mode, according to one embodiment of the present invention. FIGS. 12C and 12D depict reordering of card to separate sibling cards in response to user input, according to one embodiment of the present invention. FIG. 13A depicts the display screen in full-screen mode wherein eight cards are open, one of which has focus, according to one embodiment of the present invention. FIG. 13B depicts the display screen in card mode wherein eight cards are open, including four cards forming a group, according to one embodiment of the present invention wherein the grouped cards are depicted in a stack. FIG. 13C depicts the display screen of FIG. 13B, wherein the cards forming the group have been reordered, according to one embodiment of the present invention. FIG. 13D depicts the display screen in card mode wherein eight cards are open, including four cards forming a group, according to one embodiment of the present invention wherein the grouped cards are depicted using a distinctive vertical offset and size. FIG. 13E depicts the display screen in shuffle mode wherein eight cards are open, including four cards forming a group, according to one embodiment of the present invention wherein the grouped cards are depicted using a distinctive vertical offset. FIGS. 13F and 13G depict an example of a transition from view in which one of the members of a group has focus to a view in which the card having focus is not part of a group, according to one embodiment of the present invention. FIGS. 13H through 13K depict an example of creation of a new card group and adding cards to the group, according to one embodiment. FIG. 14 is a flow diagram depicting a method for presenting cards in grid mode according to one embodiment. FIG. 15 is a flow diagram depicting a method for card rearrangement according to one embodiment. FIGS. 16A through 16D depict a sequence for adding a card to a group according to one embodiment of the present invention. FIGS. 17A through 17D depict a sequence for removing a card from a group according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS In the following description, reference is made herein to “activities”. In the context of the present invention, an “activity” is intended to include a software application, a task, a document, a page, and/or any other activity that can be performed in connection with an electronic device. As described herein, the present invention provides techniques for displaying and manipulating representations of activities on an electronic device, using a card metaphor. Thus, activities are represented by “cards”, which is a term used herein to refer to areas of the screen that can be moved and manipulated by the user. Users interact with cards to launch, terminate, and reorder activities, as well as to navigate among activities and perform other operations, as described in more detail below. System Architecture In various embodiments, the present invention can be implemented on any electronic device, such as a handheld computer, personal digital assistant (PDA), personal computer, kiosk, cellular telephone, and the like. For example, the invention can be implemented as a feature of an application management paradigm for a software application or operating system running on such a device. Accordingly, the present invention can be implemented as part of a graphical user interface for controlling software on such a device. In various embodiments, the invention is particularly well-suited to devices such as smartphones, handheld computers, and PDAs, which have limited screen space and which are capable of running several software applications concurrently. One skilled in the art will recognize, however, that the invention can be practiced in many other contexts, including any environment in which it is useful to switch easily and smoothly from one activity to another. Accordingly, the following description is intended to illustrate the invention by way of example, rather than to limit the scope of the claimed invention. Referring now to FIG. 1, there is shown an example of an example of a device 100 including a display screen 101 showing a launcher including several icons 104 representing applications that can be opened by the user, according to one embodiment. A quick-launch area 105 is also shown, including additional icons 106 for launching commonly used applications and other functions. Quick-launch area 105 is not necessary for the present invention, but is shown for illustrative purposes only. In one embodiment, screen 101 is touch-sensitive, so that a user can activate an application or function by touching a displayed icon 104, 106. In one embodiment, device 100 also includes a touch-sensitive gesture area 102 for entering gesture-based commands, as described in the above-referenced related application. In another embodiment, screen 101 and/or gesture area 102 is sensitive to user motion in proximity to the surface, so as to detect gestures and commands performed near the surface but without necessarily contacting the surface. For illustrative purposes, device 100 as shown in FIG. 1 is a personal digital assistant or smartphone. Such devices commonly have telephone, email, and text messaging capability, and may perform other functions including, for example, playing music and/or video, surfing the web, running productivity applications, and the like. The present invention can be implemented, according to various embodiments, in any type of device having a touch-sensitive screen, and is not limited to devices having the listed functionality. In addition, the particular layout shown in FIG. 1 is merely exemplary and is not intended to be restrictive of the scope of the claimed invention. In various embodiments, touch-sensitive screen 101 and gesture area 102 can be implemented using any technology that is capable of detecting a location of contact and/or user motion proximate to the surface. Alternatively, the present invention can be implemented with other user input mechanisms, such as a keyboard, trackball, stylus, or the like. One skilled in the art will recognize that the techniques of the present invention are well suited to direct manipulation, but that any input technique can be used. In one embodiment, gesture area 102 provides a touch-sensitive control area extending beyond the edges of touch-sensitive screen 101, as described in related application Ser. No. 12/115,992, filed May 6, 2008, for “Extended Touch-Sensitive Control Area for Electronic Device”, the disclosure of which is incorporated herein by reference. In one embodiment, device 100 as shown in FIG. 1 also has a physical button 103. In one embodiment, physical button 103 can be used to toggle between full-screen mode and card mode, as described in more detail below. Physical button 103 is not needed for the present invention, and is shown for illustrative purposes only. Card Metaphor As described herein, in one embodiment two or more modes are available for interacting with device 100. A determination is made as to which display mode is currently active on device 100. Depending on the current display mode, cards 301 are displayed differently. In various embodiments, display modes can include any combination of the following: a “full-screen” mode, in which a card representing an application or other activity occupies substantially the entire display screen 101; a “card” mode, in which at least two cards are presented, at least one of which is only partially visible, with each card representing an application or other activity; a “shuffle” mode, in which cards are presented at a reduced size with respect to card mode, so that more than one card can be presented in full; and a “grid” mode, in which card size is reduced according to available screen space. As described in more detail below, device 100 can receive input from the user. In various embodiments, such input can cause device 100 to switch from one display mode to another, and/or to reposition displayed cards in response to user input specifying a switch to another card. One skilled in the art will recognize that the names given to the modes (“full-screen”, “card”, “shuffle”, and “grid”) are used herein solely for identification purposes, and are not intended to connote any particular characteristic of the modes or restrict the scope of the invention in any other way. In general, and without limiting the scope of the claimed invention: full-screen mode is optimized for maximum use of screen space for interacting with a single activity; card mode is optimized for easily switching focus from one activity to another and for easily initiating and terminating activities; shuffle mode is optimized for reordering and manipulating cards representing activities; and grid mode is optimized to provide an overview of open activities. Many actions can be taken in more than one mode. In one embodiment, activities are live and able to accept input and display output regardless of the current mode; in another embodiment, activities may be live in some modes but not in others. The various modes will be described in more detail below. Full-Screen Mode Referring now to FIG. 2, there is shown display screen 101 in full-screen mode according to one embodiment. In the example, a calendar application occupies substantially the entire display screen 101; however, one skilled in the art will recognize that any activity can be presented in this manner. As shown in FIG. 2, even in full-screen mode, certain portions of display screen 101 may be reserved for a title bar, battery indicator, clock, signal strength indicator, and the like. Accordingly, the term “full-screen mode” as used herein is intended to refer to any arrangement wherein the primary focus of the screen 101 is a single activity, even if other, smaller areas of screen 101 are reserved for other functions. The user interacts with the active activity (such as the calendar application shown in FIG. 2) by conventional user interface methods, including manipulating on-screen elements, entering text, and the like. Such techniques are well known in the art. Card Mode Referring now to FIG. 3, there is shown an example of display screen 101 in card mode according to one embodiment. In one embodiment, the user can switch among modes pressing physical button 103. In another embodiment, the user can touch or double-tap a card 301 in card mode to initiate a switch to full-screen mode. In yet another embodiment, device 100 switches from card mode to full-screen mode after some period of inactivity. Other mechanisms can also be provided for switching among the modes. As can be seen in FIG. 3, in one embodiment in card mode, card 301B having focus is fully visible. Other cards 301A, 301C are only partially visible and do not have focus. Usually, card 301B having focus represents the same activity that was displayed in full-screen mode immediately before the switch to card mode. Any number of partially displayed cards 301A, 301C can be presented. In the example of FIG. 3, two such cards 301A, 301C are shown, one on either side of card 301B having focus. Typically, in an arrangement where cards 301 are presented along a linear axis, one or two partially displayed card(s) 301 can be displayed adjacent to the card 301 that has focus. Also, as shown in FIG. 3, in one embodiment, partially displayed cards 301A, 301C are shown slightly smaller than card 301B having focus, so as to further emphasize the fact that card 301B has focus. In one embodiment, partially displayed cards 301A, 301C provide a positional context for card 301B, and provide a mechanism for navigating to other activities. In one embodiment, the user can designate a card 301A, 301C to have focus by touching any area within the card; this causes the designated card 301 to move to the central portion of display screen 101, and causes other cards to move as well, so that the same positional sequence is maintained. In one embodiment, the user can designate a card 301A, 301C to have focus by dragging any displayed card 301 to cause the desired card 301 to move to the central portion of display screen 101; again, other cards 301 move as well, so that the same positional sequence is maintained. In one embodiment, the user can move cards 301 left and right by dragging a finger along gesture area 102. In one embodiment, such a gesture can be performed even in full-screen mode, causing cards 301 to move left and right in the same manner as described above for card mode. In response to a user command, cards 301 shift position on screen 101, so that, for example card 301B that currently has focus becomes only partially visible and one of the other cards 301A, 301C becomes fully visible and has focus. The cards 301 move in concert with each other, in response to user commands. Thus, for example, if the user indicates that a central card 301 should move in a rightward direction, then the entire display shifts to the right, as follows: The central card 301 (which has focus) moves to the right, so that it is only partially visible along the right hand side of display screen 101. If any card 301 was partially visible to the right of the card 301 that has focus, it moves off the right edge of display screen 101 so that it is no longer visible. If any card 301 was partially visible to the left of the card 301 that has focus, it moves to the right so that it is now fully visible and has focus. In one embodiment, the user indicates such movement, for example, by direct manipulation of the displayed cards 301. This can be performed on a touch screen, for example, by dragging the central card 301 to the left or to the right on the screen or by tapping on a card that is partially displayed. Alternatively, a trackball, touch-sensitive pad, or other input device can be provided for facilitating such direct manipulation. Accordingly, in one embodiment, when the user drags a card 301 to the left or right within display screen 100, other cards 301 move in concert with the dragged card 301. Thus, for example, if the user drags card 301B to the right, cards 301A and 301C move to the right as well. This would cause card 301C to move off the screen, and card 301A would move to the central position of display screen 100. The same result would occur if the user drags card 301A to the right. In one embodiment, if the user drags cards 301 so as to place a card 301 sufficiently close to the central position to make it clear that the intention is to give the card 301 focus, the card 301 snaps into the central position upon completion of the drag operation. Other cards 301 snap into place accordingly to maintain the same positional sequence. Thus, the user need not drag the card 301 all the way to the central position. If, on the other hand, the user drags cards 301 a small amount that does not cause a new card 301 to be sufficiently close to the central position, all cards 301 snap back to their previous positions upon completion of the drag operation. In one embodiment, cards 301 show applications or other activities in operation. Thus, as the user navigates among cards 301, he or she can see the actual live application or other activity within each card 301. In various embodiments, card motion can be constrained to a single axis, for example horizontal or vertical but not both, so as to simplify the positional relationship among cards 301. Alternatively, two or more axes of movements can be made available. In the examples described herein, cards 301 are arranged in a horizontal row. The user moves cards 301 along a horizontal axis to navigate from one activity to another. One skilled in the art will recognize that other arrangements are possible. For example, cards 301 could be arranged vertically instead of horizontally. Alternatively, a two-dimensional arrangement of cards 301 can be provided. In one embodiment, once the user has moved the desired card 301 to the central position so that it has focus, he or she can indicate that display screen 101 should return to full-screen mode, so that the card 301 having focus occupies substantially the entire display screen 101. Alternatively, device 100 can return to full-screen mode automatically after a period of time elapses where no card movement takes place, or if the user starts to interact with the activity that has focus. In one embodiment, the card 301 that currently has focus is always fully visible and displayed in a substantially central location on display screen 101 with respect to partially displayed cards 301. Referring now to FIG. 4, there is shown an example of display screen 101 in card mode, after the user has caused card 301C (representing an email application) to become centrally located and to have focus in one embodiment. As can be seen in FIG. 4, card 301B that previously had focus is now partially visible to the left of card 301C. Card 301A is no longer visible. In one embodiment, the user can touch or double-tap card 301C to re-enter full-screen mode with the new activity having focus and being active, as shown in FIG. 5. Alternatively, the user can press button 103 to toggle between full-screen mode and card mode. Thus, in FIG. 5 the user has completed the switch from the calendar application of FIG. 2 to the email application. Referring now to FIG. 7, there is shown a conceptual depiction of a sequence for switching from one activity to another according to one embodiment of the present invention. A number of activities are open, each represented by a card 301A through 301H. Screen 101 only shows a subset of the cards 301 at any given time. As can be seen in the Figure, a positional sequence is maintained even though most of the cards 301 are not visible at any given time. FIG. 7 depicts movement of the display screen 101 from an initial position where card 301B has focus (and cards 301A and 301C are partially visible) to a second position where card 301E is has focus (and cards 301D and 301F are partially visible). The movement of display screen 101 is conceptual. In actual operation in one embodiment, the switch from card 301B to card 301E is accomplished by moving cards 301 in a leftward direction on screen 101. For example, the user can repeatedly drag cards 301 in a leftward direction to cause cards 301C, 301D, and 301E to successively occupy the central position. Alternatively, the user can click on partially displayed card 301C to move it to the central position and to cause card 301D to be partially displayed, then on partially displayed card 301D to move it to the central position and to cause card 301E to be partially displayed, and then on partially displayed card 301E to move it to the central position. Either mechanism operates to shift focus from the activity represented by card 301B to the activity represented by card 301E. In one embodiment, a transition effect is performed to enhance the smoothness of the transition as cards 301 move and/or snap into place. In one embodiment, the card 301 that is centrally displayed at any given time is shown slightly larger than other cards 301. In general, the sequence of cards 301 is persistent, even when the user moves from one card to another 301. In one embodiment, the sequence is circular, so that the rightmost card 301 is considered to be to the left of the leftmost card 301. Thus, moving off one end of the sequence takes the user back to the other end of the sequence. In another embodiment, as depicted in FIG. 4, the sequence is not circular, so that there is a first and last card 301 in the sequence. In one embodiment, cards 301 may be moved within the sequence automatically in some circumstances. For example, frequently used cards 301 can be moved to more prominent locations in the sequence, while less frequently used cards 301 can remain in less prominent locations. Referring now to FIGS. 6A through 6F, the operation of the invention according to one embodiment is further illustrated by way of an example of a sequence for switching from one activity to another; specifically, the user switches from a calendar application to a telephone application. FIG. 6A shows an initial state in which device 100 shows a calendar application in full-screen mode. In FIG. 6B, the user has switched to card mode, for example by pressing button 103. Now, the calendar application is visible in card 301B, and the telephone application is partially visible in card 301A. FIGS. 6C through 6E show the effect of the user dragging card 301B in a rightward direction. As the user drags card 301B, card 301A also moves in a rightward direction, until it is centrally located in display screen 101, as shown in FIG. 6E. As discussed above, this motion can also be initiated in response to the user touching the visible portion of card 301A in FIG. 6B, or by dragging in a horizontal rightward direction in gesture area 102. Once the telephone application is in the central position in card 301A as shown in FIG. 6E, the user can tap on card 301A or press button 103 to switch to full-screen mode, as shown in FIG. 6F. In one embodiment, the user can also interact with the telephone application directly in card mode as shown in FIG. 6E. In one embodiment, the user can launch additional activities by returning to the launch screen as shown in FIG. 1. In one embodiment, this is performed by performing a gesture or other command. In one embodiment, the user can also launch additional activities by touching an icon 106 in quick-launch area 105. In one embodiment, newly launched activities generate new cards that are added to the end of the sequence of cards shown in FIG. 7. In one embodiment, the card for the newly launched activity is, by default, given focus and presented in the central position on screen 101. In one embodiment, the user can dismiss activities, for example by closing a card 301. In one embodiment, a card 301 can be closed by clicking on a control within the card. In another embodiment, a card 301 can be closed by dragging the card upward off screen 101, or performing a flicking action in an upward direction. Referring now to FIGS. 8A and 8B, there is shown an example of a sequence where the user drags card 301B upward off screen 101, causing card 301B to be dismissed. As can be seen in FIGS. 8A and 8B, in one embodiment dismissing card 301B causes 301C to move leftward to fill in the gap caused by card 301B being dismissed. In general, when a card 301 is dismissed, other cards 301 move to fill the gap. In one embodiment, dismissing a card 301 causes the application associated with the card to close. In another embodiment, the application remains open even when card 301 is closed. In one embodiment, a card 301 might close automatically in certain circumstances. For example, if a task associated with an application is completed, or if an application crashes or terminates, the card 301 for the application can close automatically. In one embodiment, the user can also rearrange cards 301 while in card mode, by tapping and holding a card 301 for a period of time (such as, for example, 500 milliseconds), and then dragging a card 301 from one position to another. An example is shown in FIGS. 9A through 9D. The user taps and holds his or her finger on card 301B, then drags card 301B upward, causing cards 301C and 301D to move to fill the gap. The user does not immediately release card 301B however; instead, he or she moves card 301B into a position between cards 301C and 301D, as shown in FIG. 9C. As the user moves the dragged card 301B between two other cards 301C and 301D, the two cards 301C and 301D separate to allow placement of the dragged card 301B. The result is the sequence shown in FIG. 9D, where card 301B is now positioned between cards 301C and 301D. In one embodiment, some visual feedback is provided to indicate that card rearrangement is in progress; for example, the displayed cards 301 can be highlighted in some unique manner, or can include an animation or other effect, once the user has held his or her finger in the appropriate location for at least the specified period of time. Once the movement action has been initiated, the user can continue to drag card 301 in any direction in order to place card 301 at the desired position within the sequence of cards 301. The sequence of cards 301 scrolls left or right in response to the dragging operation; for example, if the user drags card 301 near the right edge of the screen, the sequence scrolls to the left to cause additional cards 301 in the sequence to be shown while others scroll off the screen. When the user releases card 301, ending the drag operation, card 301 is placed at the indicated position, thus completing the card sequence rearrangement action. Shuffle Mode In one embodiment, an additional mode, referred to as “shuffle” mode, provides the user with another mechanism for viewing and rearranging cards. In one embodiment, in shuffle mode, cards 301 are shown at a reduced size, thus permitting more cards 301 to be shown on screen 101 simultaneously. In one embodiment, when device 100 is in shuffle mode, cards 301 are arranged linearly as they are in card mode. The user can rearrange cards 301 as described above for card mode, and substantially all of the behaviors described above with respect to card mode also apply to shuffle mode. Thus, when the user drags a card 301 to the left or right within display screen 100, other cards 301 move in concert with the dragged card 301. Shuffle mode provides a way to rearrange cards while being able to view more cards 301 simultaneously. A comparison of FIGS. 10A and 10B provides an example of the differences between card mode and shuffle mode. Referring now to FIG. 10A, there is shown display screen 101 in card mode according to one embodiment of the present invention and as described above; here, seven cards 301 are currently open, although only one card 301D is shown in full, with portions of two adjacent cards 301C, 301E also being shown. Cards 301A, 301B, 301F, and 301G are shown in FIG. 10A for illustrative purposes to depict their positional relationship with visible cards 301C, 301D, 301E, even though cards, 301B, 301F, and 301G are off-screen and not currently visible on screen 101. Referring now to FIG. 10B, there is shown display screen 101 in shuffle mode for the same seven cards 301 that were included in FIG. 10A. Here, three cards 301C, 301D, and 301E are shown in full, with portions of two adjacent cards 301B, 301F also being shown. In one embodiment, the sizes of the cards are reduced so that the additional displayed cards fit on screen 301. One skilled in the art will recognize that the particular number of cards 301 visible on the screen can vary from one embodiment to another, and that the particular display of three fully visible cards, plus two partially visible cards, is merely exemplary of one embodiment. In one embodiment, any number of cards 301 can be completely visible within screen 101. For example, as shown, three cards 301 might be completely visible, with additional partially-visible cards 301 on either side. Alternatively, five cards 301 might be completely visible, with additional partially-visible cards 301 on either side. In one embodiment, the user can interact with cards 301 in shuffle mode in substantially the same manner as in card mode. Cards 301 can be moved back and forth, and a positional relationship among cards 301 is maintained. Cards 301 can be rearranged within the sequence by dragging and dropping. A card 301 can be closed, and its activity dismissed, by dragging the card 301 upward off screen 101, or performing a flicking action in an upward direction, in a manner similar to the operation described above in connection with FIGS. 8A and 8B. For example, referring now to FIG. 10C, there is shown an example of an upward drag or flicking action to dismiss card 301E. Upon dismissal of card 301E, card 301F would shift to the left to occupy the space formerly occupied by card 301E, and card 301G would move to the left to maintain consistent spacing among cards. In one embodiment, while in shuffle mode, the user can initiate rearrangement of the card sequence without necessarily first performing a tap-and-hold input operation for some period of time. For example, as shown in FIG. 10D, the user can immediately initiate a rearrangement of cards 301 by dragging a card 301D downward; the user need not tap-and-hold for some period of time. If the user wishes to tap-and-hold, he or she can still initiate the rearrangement operation by doing so. In either case, once the rearrangement operation has commenced, the user can drag in any desired direction while continuing to hold his or her finger on screen 101; the display of cards 301 scrolls left or right as appropriate when the user's finger approaches the right- or left-most edge of screen 101. While card rearrangement is taking place, cards 301 shift position to fill gaps, in a manner similar to that described above in connection with FIGS. 9A through 9D for card view. Thus, in shuffle view, if the user drags card 301D downwards a sufficient amount, as shown in FIG. 10D, card 301E would move to the left to fill the gap. As the user moves the dragged card 301D between two other cards, the two cards would separate to allow placement of the dragged card 301D. In one embodiment, in shuffle mode, cards 301 show applications or other activities in operation. Thus, as the user navigates among cards 301, he or she can see the actual live application or other activity within each
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