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 DECTPU built-in procedure FILE_PARSE is called from
    TPU code, DECTPU 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.