(* booleanExpression.ml *)
type 'canonical boolean_expression = [
  `Variable of int |
  `Not of 'canonical boolean_expression |
  `And of 'canonical boolean_expression list |
  `Or of 'canonical boolean_expression list
];;

let rec string_of_boolean_expression e =
  let string_of_boolean_expression_list o l =
    match l with
      | [] -> ""
      | h :: t ->
        let rec loop l s =
          match l with
            | [] -> s
            | h :: t ->
              loop t (s ^ o ^ (string_of_boolean_expression h))
        in "(" ^ (string_of_boolean_expression h) ^ (loop t "") ^ ")"
  in
  match e with
    | `Variable( i ) -> string_of_int i
    | `Not( sub_e ) -> "¬" ^ (string_of_boolean_expression sub_e)
    | `And l -> string_of_boolean_expression_list "∧" l
    | `Or l -> string_of_boolean_expression_list "∨" l
;;

type not_canonical_boolean_expression = [`Not_canonical] boolean_expression;;

(* booleanExpression.mli *)
type not_canonical_boolean_expression = [
  `Variable of int |
  `Not of not_canonical_boolean_expression |
  `And of not_canonical_boolean_expression list |
  `Or of not_canonical_boolean_expression list
];;

val string_of_boolean_expression : not_canonical_boolean_expression -> string