P85 (**) Graph isomorphism

    Two graphs G1(N1,E1) and G2(N2,E2) are isomorphic if there is a
    bijection f: N1 -> N2 such that for any nodes X,Y of N1, X and Y
    are adjacent if and only if f(X) and f(Y) are adjacent.

    Write a predicate that determines whether two graphs are
    isomorphic. Hint: Use an open-ended list to represent the function