docs: function _detect(eachFn, arr, iterator, mainCallback) {
    eachFn(arr, function (x, index, callback) {
      iterator(x, function (v, result) {
        if (v) {
          mainCallback(result || x);
          mainCallback = async.noop;
        } else {
          callback();
        }
      });
    }, function () {
      mainCallback();
    });
  }