node f(x : bool) returns (y : bool)

contract
var c, cx : bool;
let
  c = not y;
  cx = not (x & false fby x);
tel
assume cx
enforce c

let
  automaton
    state A do y = x unless x then B
    state B do y = false fby x until x then A
  end
tel

node g(x : bool) returns (y : bool)

contract
var ok : bool;
let
  automaton
    state No_x do ok = true until x then One_x
    state One_x do ok = not x
    until x then Error
        | not x then One_no_x
    state One_no_x do ok = not x
    until x then Error
        | not x then No_x
    state Error do ok = false
  end
tel
assume ok
enforce not y

let
  y = f(x);
tel