Domains
    int=integer
    str=string
    list=int*

Predicates
    nondeterm playOn(str, str)
    nondeterm isQuartet(str, str, str, str)

Clauses
    playOn("Monkey", "Violin").
    playOn("Donkey", "Alto").
    playOn("Goat", "Violoncello").
    playOn("Bear", "Contrabass").

    isQuartet(X, Y, Z, W):-
        playOn(X, "Alto"),
        playOn(Y, "Violin"),
        playOn(Z, "Violoncello"),
        playOn(W, "Contrabass"),
        X<>Y, Z<>W, X<>Z, Y<>W, X<>W, Y<>Z.
Goal
    isQuartet(X, Y, Z, W).