Constructing Data Structures

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Some couples go out for dinner if...

go_out_for_dinner(Ma, Mb):-
married(Ma),
married(Mb),
spouse(Ma, A),
spouse(Mb, B),
are_friends(A, B).

?- go_out_for_dinner(A, B).
A=couple(peter,anna), B=couple(mark,kathleen) ; A=couple(peter,anna), B=couple(alvin,marcia) ; A=couple(mark,kathleen), B=couple(peter,anna) ; A=couple(alvin,marcia), B=couple(peter,anna)