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