line_insert...

      tag: <string>
      open flag: <yes/no>
      indent: <number>

 This inserts an empty fold line into the current buffer. The new fold is
 given the supplied tag and is made open or closed, depending on the open
 flag.

 An open fold looks like :-

      {{{ open fold tag
          contents of fold
      }}}

 A closed fold looks like :-

      ... closed fold tag

 The fold is indented from the left edge of the current fold by the indent
 given.

 eg:  line_insert... "low level I/O" yes 8