1/* @(#)xgrun.pl	24.1 2/23/88 */
    2
    3/* 
    4	Copyright 1986, Fernando C.N. Pereira and David H.D. Warren,
    5
    6			   All Rights Reserved
    7*/
    8:- mode terminal(?,+,?,+,?),
    9        gap(+),
   10        virtual(+,+,?).
   11
   12terminal(T,S,S,x(_,terminal,T,X),X).
   13terminal(T,[T|S],S,X,X) :-
   14   gap(X).
   15
   16gap(x(gap,_,_,_)).
   17gap([]).
   18
   19virtual(NT,x(_,nonterminal,NT,X),X)