(* sat.ml *)
open BooleanExpression;;

let e = `And( [`Or([`Var( 1 ); `Var(3)]);`Var( 0 );`Var(3)] ) in
print_string (string_of_boolean_expression e);;
print_newline();

(* booleanExpression.ml *)
type 'canonical boolean_expression = [
  `Var of int |
  `Not of 'canonical boolean_expression |
  `And of 'canonical boolean_expression list |
  `Or of 'canonical boolean_expression list
  constraint 'canonical = [ < `Canonical | `Not_canonical ]
];;

let rec number_of_variables e =
  match e with
    | `Var i -> i
    | `Not t -> number_of_variables t
    | `And l | `Or l -> List.fold_left (fun m t -> max m (number_of_variables t)) 0 l
;;

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
    | `Var( 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
;;

(* booleanExpression.mli *)
type 'canonical boolean_expression constraint 'canonical = [< `Canonical | `Not_canonical ]

val string_of_boolean_expression : _ boolean_expression -> string

(* output *)
File "sat.ml", line 4, characters 43-44:
Error: This expression has type
         [> `And of [> `Or of [> `Var of int ] list | `Var of int ] list ]
       but an expression was expected of type
         [< `Canonical | `Not_canonical ]
         BooleanExpression.boolean_expression