line_insert:::

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

 This inserts an empty virtual 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 :-

      >>> filename open fold tag

 A closed fold looks like :-

      ::: filename closed fold tag

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

 When a virtual fold is entered, a new buffer is created, loaded and selected.

 eg:  line_insert::: "error.log" no 0

 On DOS and OS/2 the filename has $(ENVIRONMENT_VARIABLE) strings expanded
 within it. Thus the following would be sensible.

      >>> $(INCLUDE)\sys\stat.h