/* file: /230/pl/timeflies.pl */
/* natural language parsing with difference lists */

    s(S,S0)     :- np(d(S,S1)), vp(d(S1,S0)), print(S1), nl.

    np(d(S,S0)) :- a(d(S,S1)), n(d(S1,S0)).
    np(d(S,S0)) :- n(d(S,S1)), n(d(S1,S0)).  /* noun cluster */
    np(N)       :- n(N).

    vp(d(S,S0)) :- v(d(S,S1)), pp(d(S1,S0)).
    vp(d(S,S0)) :- v(d(S,S1)), np(d(S1,S0)).
    vp(V)       :- v(V).

    pp(d(S,S0)) :- p(d(S,S1)), np(d(S1,S0)).

    n(d([time|S],S)).
    n(d([flies|S],S)).
    n(d([arrow|S],S)).
    v(d([like|S],S)).
    v(d([flies|S],S)).
    a(d([an|S],S)).
    p(d([like|S],S)).

    q     :- s([time, flies, like, an, arrow],[]).
    q1(X) :- s([time, X, like, an, arrow],[]).

/* the sentence is ambiguous (but the second meaning is tricky) */