- prolog factoids from "Relativity" on Futility Closet

relativity/facts.pl

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+% first things first, the gender of the cast players
+male(dad).
+male(man).
+male(boy).
+male(boytwo).
+female(mum).
+female(widow).
+female(stepdot).
+
+% I married the widow
+married(man, widow).
+% my father ...
+father(dad, man).
+% ... married the stepdaughter
+daughter(stepdot, widow).
+married(dad, stepdot).
+
+% stepdaughter has a boy
+son(boy,stepdot).
+
+% my wife also has a boy
+son(boytwo,widow).
+%
+% implicit in the story
+mother(mum, man).
+
+% direct child-parent relationships
+isdirectchild(Child,Parent) :- son(Child,Parent).
+isdirectchild(Child,Parent) :- daughter(Child,Parent).
+isdirectchild(Child,Parent) :- father(Parent,Child).
+isdirectchild(Child,Parent) :- mother(Parent,Child).
+
+% indirect child-parent relationships
+ischild(Child,Parent) :- isdirectchild(Child,Parent).
+ischild(Child,Parent) :- isdirectchild(Child,Random), ismarried(Random,Parent).
+
+% and the reverse
+isparent(Parent,Child) :- ischild(Child,Parent).
+
+ismarried(Man,Woman) :- married(Man,Woman).
+ismarried(Man,Woman) :- married(Woman,Man).
+
+isfather(Man,Child) :- ischild(Child,Man), male(Man).
+isfather(Man,Child) :- ischild(Child,Woman), ismarried(Woman,Man), male(Man).
+
+ismother(Woman,Child) :- ischild(Child,Woman), female(Woman).
+ismother(Woman,Child) :- ischild(Child,Man), ismarried(Woman,Man), female(Woman).
+
+isgrandfather(X,Y) :- ischild(Z,X), ischild(Y,Z), male(X).
+isgrandmother(X,Y) :- ischild(Z,X), ischild(Y,Z), female(X).
+
+isbrother(X,Y) :- male(X), ischild(X,Z), ischild(Y,Z), X \== Y.
+issister(X,Y) :- female(X), ischild(X,Z), ischild(Y,Z), X \== Y.
+
+isaunt(X,Y) :- female(X), issister(X,Z), ischild(Y,Z).
+isuncle(X,Y) :- male(X), isbrother(X,Z), ischild(Y,Z).
+
+% brotherinlaw B is married to Ps sister
+isbrotherinlaw(B,P) :- male(B), issister(Z,P), ismarried(B,Z).
+
+% motherinlaw M is mother of Ps spouse
+ismotherinlaw(M,P) :- female(M), ismarried(Z,P), ismother(M,Z).
+
+% daughterinlaw D is a female married to one of Ps children
+isdaughterinlaw(D,P) :- female(D), ischild(X,P), ismarried(X,D).