Description

   The FILE_SEARCH user routine allows an application to replace
   the TPU$FILE_SEARCH routine with its own file-searching routine.
   The calling program passes the address of the routine to the
   TPU$INITIALIZE routine using the TPU$_FILE_SEARCH item code.

   When the TPU built-in procedure FILE_SEARCH is called from TPU
   code, TPU calls either the user-written FILE_SEARCH routine
   (if one was passed to TPU$INITIALIZE) or the TPU$FILE_SEARCH
   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 handlers,
   errors in the user-written FILE_PARSE routine should be signaled
   using the TPU$SIGNAL routine.