Relations

Families provide many examples of relations between individuals in both the familial and mathematical senses – mother, father, parent, child-of, sibling, grandparent, aunt, uncle, and so on. Siblings have the same parents (one could define sibling as having at least one parent in common). An aunt or uncle is the sibling of a parent (ie., directly) or is the spouse of an aunt or uncle (i.e., by marriage). However, unless care is taken this allows 'X' to be a sibling of his or herself and potentially gives multiple ways for X to be someone's aunt or uncle. The solution is to insist that siblings, while sharing the same parents, must be different individuals; see sibling(.,.) below:

parents(william, diana, charles).
parents(henry,   diana, charles).
parents(beatrice, sarah, andrew).
parents(eugenie,  sarah, andrew).
parents(louise, sophie, edward).
parents(james,  sophie, edward).

parents(diana,   frances,   edwardS).
parents(charles, elizabeth, philip).
parents(anne,    elizabeth, philip).
parents(andrew,  elizabeth, philip).
parents(edward,  elizabeth, philip).

married(diana, charles).
married(camilla, charles). {later}
married(anne, mark).
married(sarah, andrew).
married(sophie, edward).
married(elizabeth, philip).
married(frances, edwardS). {above are data, below are rules}

parent(C,M) <= parents(C,M,D). {mother is a parent}
parent(C,D) <= parents(C,M,D). {father is a parent}

equal(X, X).
sibling(X,Y) <= parents(X,M,D) and parents(Y,M,D)
   and not equal(X,Y). {<--necessary}

aORuDirect(C, A) <= parent(C,P) and sibling(P,A).
aORuMarr(C, A)   <= aORuDirect(C,X) and married(X,A).
aORuMarr(C, A)   <= aORuDirect(C,X) and married(A,X).
aORu(C,A)        <= aORuDirect(C,A).
aORu(C,A)        <= aORuMarr(C,A).

? aORu(william, A).

-- The House of Windsor --

Comment out 'and not equal(X,Y)' above and see what happens. How can 'aORu(.,.)' be used to find nieces and nephews of an individual?

There are more Prolog examples here.