13.9.2 Family Example (descendants)

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, 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