(* 
 In the product the sum of the leaves is multiplied by the integer
 and added to the weight of the tree
*)
Definition prod2list l1 l2 :=
  fold_left plus
    (map2 (fun a b => a * sum_leaves f b + weight_tree f b) l1 l2) 0.

Theorem prod2list_eq : forall a b c l1 l2 l3 l4 l5 l6, length l1 = length l4 -> length l2 = length l5 -> length l3 = length l6 -> prod2list (l1 ++ a :: l2 ++ a :: l3) (l4 ++ b :: l5 ++ c :: l6) = prod2list (l1 ++ a :: l2 ++ a :: l3) (l4 ++ c :: l5 ++ b :: l6).