But actually, it is possible to have node without output, so expression
without output clock is perfrectly possible, as in the following example :

node h(x : bool) returns ();
 var h : bool;
let
  h = false fby (not x);
tel

node f(x : bool) returns (y: bool);
var
  u:bool;
let
  y,u = (false, h(true), true) fby (true, x, h(false));
tel