Description

   This routine allows an application to replace the TPU$FILE_PARSE
   routine with its own file-parsing routine. The calling program
   passes the address of the file-parsing routine to TPU$INITIALIZE
   using the TPU$_FILE_PARSE item code.

   When the TPU built-in procedure FILE_PARSE is called from
   TPU code, TPU calls either the user-written routine (if one
   was passed to TPU$INITIALIZE) or the TPU$FILE_PARSE routine. The
   return value of the built-in procedure is the string returned in
   the result-string argument.

   To ensure proper operation of the user's ON_ERROR error handlers,
   errors should be signaled using the TPU$SIGNAL routine.