The name of an external program that will be used to format the lines selected with the gq operator. The program must take the input on stdin and produce the output on stdout. The Unix program "fmt" is such a program. If the 'formatexpr' option is not empty it will be used instead. Otherwise, if 'formatprg' option is an empty string, the internal format function will be used C-indenting. 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.

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