Prolog not unique elements in predicate call -

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in prolog script, have defined:

mother(x,y) :-     parent_of(x,y),     female(x).

i want know if there mothers more 2 children, run:

mother(x,y), mother(x,z)

with result:

x = pam, y = m, m = bob

which has left me quite baffled.... figured if add

not(y = z)

this fix it, unsure why...

if execute query like

mother(x,y).

the result bring mothers have 2 children well.

so if database like

female(maria). female(irini). parent_of(maria,nick). parent_of(maria,dario). parent_of(irini,dewey).

and executed mother(x,y). query, result bring back

1 ?- mother(x,y). x = maria, y = nick ; x = maria, y = dario ; x = irini, y = dewey.

so result have mother (maria) has 2 children.

if only want mother 2 children, should modify mother query as:

mother(x,y) :-     parent_of(x,y),     parent_of(x,m),     y \= m,     female(x).

the result of query be:

3 ?- mother(x,y). x = maria, y = nick ; x = maria, y = dario ; false.

(false means prolog did't find more results).


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