47. The method of claim 15 , wherein the hierarchical tree is a first hierarchical tree displayed in a first folder, the first hierarchical tree having the plurality of expressions displayed at a parent level and the plurality of results displayed at a child level, the method further comprising: displaying a second hierarchical tree in a second folder having the plurality of expressions displayed at the child level and the plurality of results displayed at the parent level.
15. A method for displaying usage history of a calculator, the calculator having a user interface, the method comprising: receiving a first mathematical expression in an expression entry area on the calculator: displaying a result in a first window on the calculator, wherein the result displayed in the first window comprises a result for the first mathematical expression; displaying, in a second window on the calculator, a usage history comprising the first mathematical expression, a second mathematical expression, and results, wherein the results displayed in the second window comprise the result for the first mathematical expression and a result for the second mathematical expression, wherein the usage history is displayed in a hierarchical tree such that a result for a given mathematical expression is displayed on a different hierarchical level than the given mathematical expression; and displaying organizational tabs for the second window, wherein each organizational tab is operable to display a given category of data associated with the usage history, wherein the hierarchical tree is expandable and collapsible, and wherein the hierarchical tree can be expanded and collapsed while the result is displayed in the first window.