An F model of Schur's Lemma (CSPlib prob015)

balls: {int}; boxes: {int}; var Schur:balls -> boxes; solve { forall(i,j,k in balls: i+j=k)         not(Schur(i) = Schur(j) = Schur(k))};

this model appears in: Brahim Hnich, "Function Variables for Constraint Programming"