code: function () { $assertEveryCalled (function (early, lately) {
          
          var compo = $singleton (Component, {
                          hasMessage: $barrier () })
  
          compo.hasMessage (function (_msg) { $assert (_msg, 'mkay'); early () })
          compo.hasMessage ('mkay')
          compo.hasMessage (function (_msg) { $assert (_msg, 'mkay'); lately () }) }) }
docs: /*  Another variation of stream, having 'memory fence / memory barrier' semantics,
  widely known as synchronization primitive in concurrent programming.
  
  1.  At first, barrier is in closed state, putting any callback passed to it
  to a queue.
  
  2.  When barrier is called with value argument, it state changes to 'opened',
  triggering all queued callbacks with that value argument passed in.
  
  3.  After barrier had opened, any futher callback gets called immediately
  with that value argument passed before, i.e. short-circuits.