docs: function proxy (stream, key) {
    if (key in stream) {
      debug('not proxying prop "%s" because it already exists on target stream', key);
      return;
    }
  
    function get () {
      return stream.res[key];
    }
    function set (v) {
      return stream.res[key] = v;
    }
    Object.defineProperty(stream, key, {
      configurable: true,
      enumerable: true,
      get: get,
      set: set
    });
  }