this is an example Vim has a vi compatibility mode, but when that mode is not used, Vim has many enhancements over vi.[39] However, even in compatibility mode, Vim is not entirely compatible with vi as defined in the Single Unix Specification[40] 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".[41] 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 than one swap file, ".swn", ".swm", etc.).[42][43] This feature can be disabled.[44]