code: function(index) {
              var self    = this,
                  delay   = -1;
  
              if (typeof self.mixer.config.animation.staggerSequence === 'function') {
                  index = self.mixer.config.animation.staggerSequence.call(self, index, self.state);
              }
  
              delay = !!self.mixer.staggerDuration ? index * self.mixer.staggerDuration : 0;
  
              return self.callFilters('delayGetDelay', delay, arguments);
          }
docs: Calculates the transition delay for each target element based on its index, if
  staggering is applied. If defined, A custom `animation.staggerSeqeuence`
  function can be used to manipulate the order of indices to produce custom
  stagger effects (e.g. for use in a grid with irregular row lengths).
  
  @private
  @instance
  @since   2.0.0
  @param   {number}    index
  @return  {number}