Constructing Data Structures

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We may want to know who is spouse of who

?- married(couple(peter, S)).
S = anna

?- married(couple(marcia, S)).
no

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Argument order matters, of course

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We must specify what is a spouse

spouse(couple(A, B), A).
spouse(couple(A, B), B).

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Then we can forget about the order

?- spouse(C, peter), married(C).
C = couple(peter,anna)

?- married(C), spouse(C, marcia).
C = couple(alvin,marcia)

?- spouse(C, luca), married(C).
no