% The family database in chapter 9

mother(alice, tom).
mother(alice, carol).
mother(carol, george).
mother(carol, heather).
mother(susan, hank).

father(john,   tom).
father(john,   carol).
father(fred,   george).
father(fred,   heather).
father(george, hank).

parent(X, Y) :- father(X, Y).
parent(X, Y) :- mother(X, Y).

parents(X, Y, Z) :- father(X, Z), mother(Y, Z).

grandmother(X, Y) :- mother(X, Z), parent(Z, Y).

grandfather(X, Y) :- father(X, Z), parent(Z, Y).

grandparent(X, Y) :- grandfather(X, Y).
grandparent(X, Y) :- grandmother(X, Y).

ancestor(X, Y) :- parent(X, Y).
ancestor(X, Y) :- parent(X, _Z), ancestor(_Z, Y).

ancestors(X, Z) :- setof(X, Y^ancestor(X, Y), Z).

progeny(Y, Z) :- setof(Y, X^ancestor(X, Y), Z).


% auxiliary routines

% show shows an enumerated relation, one tuple per line

show(Relation) :-
  enumerate(Relation, List),
  list(List).


% enumerate enumerates a named relation to produce a list

enumerate(Relation, List) :-
  current_predicate(Relation, Form),
  Form =.. [Relation | Args],
  (setof(Args, Form, List) ; List = []).


% list shows a list, one element per line

list([]).
list([A | X]) :-
  write(A),
  nl,
  list(X).