val usage: string -> 'a
val usage_default: int -> 'a

val main: int -> (string -> unit) array -> string list -> unit

type 'a monoid =
  {id: 'a;
   op: 'a -> 'a -> 'a;
   op_name: string;
   pp: Format.formatter -> 'a -> unit}

val fold_list: 'a monoid -> 'a list -> 'a

val mult: int monoid
val add: int monoid

val print_fold: 'a monoid -> 'a list -> unit
val print_prod: int list -> unit
val print_sum:  int list -> unit