code: function(cb) {
      cb = cb || _.noop;
      const self = this;
      self._step = 0;
      if (self._stack.length<=0) return cb();
      (function next() {
        const step = _getStep(self);
        if (!step) {
          cb();
        } else if (_.isFunction(step)) {
          step.call(self, next);
        } else if (_.isFunction(step[self._exec])) {
          step[self._exec](next);
        }
      })();
    }
docs: Avvia lo stack di elementi
  @param {function} [cb]
  @returns {*}