External program to use for "=" command. When this option is empty the internal formatting functions are used; either 'lisp', 'cindent' or 'indentexpr'. When Vim was compiled without internal formatting, the "indent" program is used. Environment variables are expanded |:set_env|. See |option-backslash| about including spaces and backslashes. This option cannot be set from a |modeline| or in the |sandbox|, for security reasons.