(* forall n > 0. d(n) = u(n) - u(n-1) and d(0) = 0 *) let node diff u = y where rec y = 0.0 -> u -. pre(u) (* forall n > 0. x'(n) = k u(n) / h) - (k u(n-1) / h) * x'(0) = 0 *) let node derivative(h)(k, u) = x' where rec x' = diff (k *. u /. h)