records. An alternative to cards was Punched tape. It could be created by some teleprinters (such as the Teletype), which used special characters to indicate ends of records. Some early operating systems included batch text editors, either integrated with language processors or as separate utility programs; one early example was the ability to edit SQUOZE source files for SCAT in SHARE Operating System. The first interactive text editors were "line editors" oriented to teleprinter- or typewriter-style terminals without displays. Commands (often a single keystroke) effected edits to a file at an imaginary insertion point called the "cursor". Edits were verified by typing a command to print a small section of the file, and periodically by printing the entire file. In some line editors, the cursor could be moved by commands that specified the line number in the file, text strings (context) for which to search, and eventually regular expressions. Line editors were major improvements over keypunching. Some line editors could be used by keypunch; editing commands could be taken from a deck of cards and applied to a specified file. Some common line editors supported a "verify" mode in which change commands displayed the altered lines. When computer terminals with video screens became available, screen-based text editors (sometimes called just "screen editors") became common. One of the earliest full-screen editors was O26, which was written for the operator console of the CDC 6000 series computers in 1967. Another early full-screen editor was vi. Written in the 1970s, it is still a standard editor on Unix and Linux operating systems. Also written in the 1970s was the UCSD Pascal Screen Oriented Editor, which was optimized both for indented source code and general text. Emacs, one of the first free and open-source software projects, is another early full-screen or real-time editor, one that was ported to many systems. A full-screen editor's ease-of-use and speed (compared to the line-based editors) motivated many early purchases of video terminals. The core data structure in a text editor is the one that manages the string (sequence of characters) or list of records that represents the current state of the file being edited. While the former could be stored in a single long consecutive array of characters, the desire for text editors that could more quickly insert text, delete text, and undo/redo previous edits led to the development of more complicated sequence data structures. A typical text