stirling(N, M, X) :- N < M, X is 0.
stirling(N, N, 1).
stirling(N, 1, 1).
stirling(N, 0, 0) :- N > 0.
stirling(N, M, X) :- N > M, I is N-1, J is M-1, stirling(I, J, K), stirling(I, M, L), X is K + M * L.
petla(N, N, X) :- stirling(N, N, K), X is K.
petla(N, I, X) :- I < N, K is I+1, stirling(N, I, J), petla(N, K, L), X is J + L.
bell(N, X) :- petla(N, 0, K), X is K.