code: function valve(pw, iv, evt) {
      if (!evt) return;
      var drip = new Drip(pw, iv, evt, pw.options.endEvent);
      drip.callback = function callback(artery) {
          if (artery !== iv.artery) return; // event may be emitted external from the current flow
          iv.drain(drip);
      };
      if (drip.pulse.event !== pw.options.endEvent) {
          iv.push(drip);
          iv.meds.push(evt);
      }
      iv.listen(drip.pulse.event, drip.callback);
      iv.dripCount += drip.pulse.event === pw.options.endEvent ? 0 : drip.pulse.repeat;
      return drip;
  }
docs: Valve that regulates the drip (pulse event) flow rate of the I.V. catheter (emitter)
  
  @private
  @arg {PulseEmitter} pw the pulse emitter
  @arg {Array} iv the I.V. catheter
  @arg {(String | Object)} evt the event string or object
  @returns {Object} the drip / pulse event w/callback function used by the I.V. catheter