code: function press_button(button) {
          if (!guac_touchscreen.currentState[button]) {
              guac_touchscreen.currentState[button] = true;
              if (guac_touchscreen.onmousedown)
                  guac_touchscreen.onmousedown(guac_touchscreen.currentState);
          }
      }
docs: Presses the given mouse button, if it isn't already pressed. Valid
  button values are "left", "middle", "right", "up", and "down".
  
  @private
  @param {String} button The mouse button to press.