/*
    sentence/1
    sentence(+S)

    The predicate sentence returns true if the list of tokens
    has a valid derivation according to the grammar rules.

    This definite clause grammar is written without the --> macro.

    Warren Sack <wsack@ucsc.edu>
    May 2017
	
*/

append([],List,List).
append([First|Rest],List2,[First|List3]) :- append(Rest,List2,List3).

sentence(S) :- append(NP,VP,S),noun_phrase(NP),verb_phrase(VP).
noun_phrase(NP) :- append(A,N,NP),article(A),noun(N).
verb_phrase(VP) :- append(V,NP,VP),verb(V),noun_phrase(NP).
article([a]).
article([the]).
noun([man]).
noun([ball]).
verb([hit]).
verb([took]).