Constructing Data Structures

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Being friends is symmetric

are_friends(A, B):- friends(A, B).
are_friends(A, B):- friends(B, A).

?- are_friends(anna, X).
X = marcia ;
X = luca

?- are_friends(X, anna).
X = marcia ;
X = luca

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Some of our friends are married

married(couple(peter, anna)).
married(couple(mark, kathleen)).
married(couple(alvin, marcia)).

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Note that married defines couples of persons

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