TPU$SIGNAL performs the same function as the Run-Time Library
    routine LIB$SIGNAL, but it also processes TPU facility messages
    to allow TPU language ON_ERROR handlers to be called.

    For example, assume that a user-written file input/output routine
    is designed to signal the error TPU$_OPENIN when it fails to
    open a file. Calling the TPU$SIGNAL routine and passing the value
    TPU$_OPENIN allows a case-style TPU ON_ERROR handler to receive
    the error, thus preserving the documented return values for TPU
    built-in procedures such as READ_FILE.

                                   NOTE

       You must call TPU$INITIALIZE before you call the TPU$SIGNAL
       routine.

    If TPU$_QUITTING, TPU$_EXITING, or TPU$_RECOVERFAIL are passed to
    the routine, it calls the Run-Time Library routine LIB$SIGNAL.

    If facility messages other than TPU messages are passed to the
    TPU$SIGNAL routine, it calls the LIB$SIGNAL routine and passes
    the appropriate condition value.