Description

   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.