/* $DOC$
   $NAME$
      ft_OnTick()
   $CATEGORY$
      Event
   $ONELINER$
      Evaluate a designated code block at a designated interval.
   $SYNTAX$
      ft_OnTick( bCode, nInterval )
   $ARGUMENTS$
      <bCode> is the code block to evaluate.
      <nInterval> is the number of clock ticks to wait between
                  evaluations of the code block.
   $RETURNS$
      NIL
   $DESCRIPTION$
      This function effectively allows you to run tasks in the background
      by transparently and periodically calling a designated routine.

      To halt the execution of the background function, call ft_OnTick()
      with no arguments.

      This function makes heavy use of several undocumented internal
      routines.  If this fact makes you uncomfortable then don't use
      this function, you quivering sack of cowardly slime.
   $EXAMPLES$
      // Set up a self-updating on-screen clock

      ft_OnTick( "CLOCK", 9 )

      PROCEDURE Clock()

         LOCAL nRow := Row()
         LOCAL nCol := Col()

         @ 0, 0 SAY Time()

         SetPos( nRow, nCol )

         RETURN
   $SEEALSO$

   $END$
 */