Composing concurrency

• Concurrency constructs in SJS are compositional
• E.g. here we use '@' on a fork-join expression:

      function promptNumber(name) {
        try {
          ... create div with text input & button elements
          ... append to body
          Ajax.DOM.waitForEvent("click", button);
          return text_input.value;
        }
        finally {
          output.removeChild(div);
        }
      }

      function clearAll() {
        Ajax.DOM.waitForEvent("click", "clearall");
        output.innerHTML = "";
      }

      while (true) {
        calculate(promptNumber("1."), promptNumber("2.")) @ 
        Ajax.DOM.waitForEvent("click", "restart") @
        clearAll();
      }