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.

(default "")