:help =={motion} Filter {motion} lines through the external program
given with the 'equalprg' option. When the 'equalprg'
option is empty (this is the default), use the
internal formatting function C-indenting and
'lisp'. But when 'indentexpr' is not empty, it will
be used instead indent-expression. When Vim was
compiled without internal formatting then the "indent"
program is used as a last resort.
