Retro allows for insertion of documentation blocks. The contents of these can be
in any format desired, though the standard Retro system uses Restructured Text for
all embedded and external documentation.

  doc{
  =============
  Function: foo
  =============

  Stack
  ----
  a1 a2 - b

  Usage
  -----
  Adds a1 to a2 returning b.
  }doc

  : foo ( aa-b ) + ;

