predicate fzn_knapsack(array[int] of int: w, array[int] of int:p, 
                       array[int] of var int:x, var int: W, var int: P) = 
    forall (i in index_set(x)) (x[i] >= 0) /\
    W >= 0 /\ P >= 0 /\
    P = sum(i in index_set(p)) (x[i]*p[i]) /\
    W = sum(i in index_set(w)) (x[i]*w[i]);