!TITLE A Small Database
!KEY
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 question

            ?- 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