sequences of keystrokes and call internal or user-defined functions and mappings. Abbreviations, similar to macros and key mappings, facilitate the expansion of short strings of text into longer ones and can also be used to correct mistakes. Vim also features an "easy" mode for users looking for a simpler text editing solution. There are many plugins available that extend or add new functionality to Vim. These plugins are usually written in Vim's internal scripting language, vimscript (also known as VimL), but can be written in other languages as well. There are projects bundling together complex scripts and customizations and aimed at turning Vim into a tool for a specific task or adding a major flavour to its behaviour. Examples include Cream, which makes Vim behave like a click-and-type editor, or VimOutliner, which provides a comfortable outliner for users of Unix-like systems. Features and improvements over vi Vim has a vi compatibility mode, but when that mode is not used, Vim has many enhancements over vi. However, even in compatibility mode, Vim is not entirely compatible with vi as defined in the Single Unix Specification and POSIX (e.g., Vim does not support vi's open mode, only visual mode). Vim's developers state that it is "very much compatible with Vi". Some of Vim's enhancements include completion functions, comparison and merging of files (known as vimdiff), a comprehensive integrated help system, extended regular expressions, scripting languages (both native and through alternative scripting interpreters such as Perl, Python, Ruby, Tcl, etc.) including support for plugins, a graphical user interface (gvim), limited integrated development environment-like features, mouse interaction (both with and without the GUI), folding, editing of compressed or archived files in gzip, bzip2, zip, and tar format and files over network protocols such as SSH, FTP, and HTTP, session state preservation, spell checking, split (horizontal and vertical) and tabbed windows, Unicode and other multi-language support, syntax highlighting, trans-session command, search and cursor position histories, multiple level and branching undo/redo history which can persist across editing sessions, and visual mode. While running, Vim saves the user's changes in a swap file with the ".swp" extension. This file can be used to recover after a crash. If a user tries to open a file and a swap file already exists, Vim will warn the user, and if the user proceeds, Vim will use a swap file with the extension ".swo" (or, if there is already more