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The goal descendant(X,Y) is true if Y is
a descendant of X.
descendant(X, Y) :- offspring(X, Y). descendant(X, Z) :- offspring(X, Y), descendant(Y, Z). offspring(abraham, ishmael). offspring(abraham, isaac). offspring(isaac, esau). offspring(isaac, jacob).
If for example the query
| ?- descendant(abraham, X).
is executed, then Prolog’s backtracking results in different descendants of Abraham being returned as successive instances of the variable X, i.e.
X = ishmael X = isaac X = esau X = jacob