type t =
    Atom of string
  | And of t * t
  | Or of t * t
  | Not of t


let rec string_of_formula = function
    |And(f1,f2) ->
            Printf.sprintf "(%s And %s)"
            (string_of_formula f1)
            (string_of_formula f2)
    |Or(f1,f2) ->
            Printf.sprintf "(%s Or %s)"
            (string_of_formula f1)
            (string_of_formula f2)
    |Not(f) -> Printf.sprintf "(Not %s)" (string_of_formula f)
    |Atom(s) -> s
;;

let print s = print_endline (string_of_formula s)