Question

I have a very simple program in swi-prolog, which in I define all predicates for family relations. I also defined some facts (based on bible’s Abraham’s family).
So I defined a predicate “mother_in_law” like this:

mother_in_law(X,Y):- female(X),mother(X,Z),married(Z,Y).

and some facts:

female(sarah).
mother(sarah,isaac).
married(rebekah,isaac).

So now, when I type in the program

?- mother_in_law(sarah,X).

it should give me only X=rebekah. But it gives me some other values for X which do not stand in the condition that I defined.

Can someone tell me what am I doing wrong?

thanks a lot

I add the whole text part:

father(X,Y). % Define Predicates%
mother(X,Y).
married(X,Y).
male(X).
female(X).
parent(X,Y).
diff(X,Y).
male_cousin(X,Y).
brother_in_law(X,Y).
mother_in_law(X,Y).
cousin_three(X,Y).
cousin(X,Y).
sib(X,Y).

%Define Facts%
male(terah). male(abram). male(nahor). male(haran). male(isaac).
male(lot). male(moab). male(ben_ami). male(bethuel). male(laban).
male(esau). male(jacob). male(reuben).
female(sarah). female(milcah). female(lot_daughter_1). female(lot_daughter_2).
female(rebekah). female(leah). female(dinah).
father(terah,abram). father(terah,nahor).  father(terah,haran).
father(abram,isaac).  father(haran,lot). father(nahor,bethuel).
father(bethuel,laban). father(laban,leah). father(isaac,jacob).
father(isaac,esau).
father(jacob,reuben).
mother(sarah,isaac). mother(milcah, bethuel). mother(rebekah,jacob).
mother(rebekah,esau). mother(leah,reuben).
mother(lot_daughter_1,moab). mother(lot_daughter_2,ben_ami).
married(abram,sarah). married(nahor,milcah).
married(isaac,rebekah). married(jacob,leah).
married(sarah,abram). married(milcah,nahor).
married(rebekah,isaac). married(leah,jacob).



%Define complicated predicates%
    parent(X,Y) :- father(X,Y).
    parent(X,Y) :- mother(X,Y).
    diff(X,Y) :- not(X=Y).
    sib(X,Y):- parent(Z,X), parent(Z,Y), diff(X,Y).  %define sibling %
    mother_in_law(X,Y):- female(X),mother(X,Z),married(Z,Y). %question 1%
    male_cousin(X,Y):- male(X),father(W,X),parent(Z,Y),sib(W,Z).    %question 2%
    cousin(X,Y):- parent(Z,X), parent(W,Y), sib(W,Z).       %question 3%
    cousin_three(X,Y):- parent(Z,X),parent(G,Z),parent(H,Y),parent(K,H),cousin(G,K).
    brother_in_law(X,Y):- male(X),married(X,Z),sib(Y,Z). %question 5-X is married to Y's sibling%
    brother_in_law(X,Y):- male(X),sib(X,Z),married(Y,Z).  %X's sibling is married to Y%
    brother_in_law(X,Y):- male(X),married(X,W),married(Y,Z),sib(W,Z).  % Y is married to the sibling of X's spouse%

Answer 1

The problem with your solution is due to the following fact

mother_in_law(X,Y).

stating that anybody is a mother in law of anybody else.

Drop

father(X,Y). % Define Predicates%
mother(X,Y).
married(X,Y).
male(X).
female(X).
parent(X,Y).
diff(X,Y).
male_cousin(X,Y).
brother_in_law(X,Y).
mother_in_law(X,Y).
cousin_three(X,Y).
cousin(X,Y).
sib(X,Y).