/*
    active_to_passive/2
    active_to_passive(+Active,-Passive)

    The predicate active_to_passive accepts the derivation of an
    active sentence and transforms it into the derivation of an
    analogous sentence in passive form.

    Example:
	    active_to_passive(sentence(noun_phrase(article(the),noun(man)),
			               verb_phrase(verb(hit),
			                           noun_phrase(article(the),
			                           noun(ball)))),
			      D).
	    returns
	                      D = sentence(noun_phrase(article(the),noun(ball)),
					   verb_phrase(auxilliary(was),
					               verb(hit),
					               prep_phrase(prep(by),
					                           noun_phrase(article(the),
					                                       noun(man)))))
	    
    Warren Sack <wsack@ucsc.edu>
    May 2017
	
*/

active_to_passive(sentence(noun_phrase(A1,X1),
                           verb_phrase(verb(X2),
		                       noun_phrase(A2,X3))),
		  sentence(noun_phrase(A2,X3),
		           verb_phrase(auxilliary(Aux),
		                       verb(X2),
                                       prep_phrase(prep(by),
		                                   noun_phrase(A1,X1))))) :-
       conjugate_past_tense(X3,be,Aux).

conjugate_past_tense(noun(N),Infinitive,PastTense) :-
       is_singular(N),
       singular_past_tense(Infinitive,PastTense).
conjugate_past_tense(noun(N),Infinitive,PastTense) :-
       is_plural(N),
       plural_past_tense(Infinitive,PastTense).
is_singular(ball).
is_singular(man).
is_plural(balls).
is_plural(men).
plural_past_tense(be,were).
singular_past_tense(be,was).