let boolean_expression_of_string str =
  let lexify str =
    let keywords = ["("; ")"; "¬"; "∧"; "∨"] in
    make_lexer keywords (Stream.of_string str)
  and parse lexer =
    let rec binary_expression =
      let rec loop left_expr =
        parser
          | [< 'Kwd "∧"; right_expr = unary_expression; s >] -> loop (And [left_expr; right_expr]) s (* Syntax error here characters 10-11 *)
          | [< 'Kwd "∨"; right_expr = unary_expression; s >] -> loop (Or [left_expr; right_expr]) s
          | [< >] -> left_expr
      in
      parser [< left_expr = unary_expression; s >] -> loop left_expr s
    and unary_expression =
      parser
        | [< 'Kwd "¬"; expr = unary_expression >] -> Not expr
        | [< 'Kwd "("; expr = binary_expression; 'Kwd ")" >] -> expr
        | [< 'Int n >] -> Var n
    in
    binary_expression lexer
  in
  parse (lexify str)
;;