code: function(property, staggerIndex, duration) {
              var self  = this,
                  delay = self.getDelay(staggerIndex),
                  rule  = '';
  
              rule = property + ' ' +
                  (duration > 0 ? duration : self.mixer.config.animation.duration) + 'ms ' +
                  delay + 'ms ' +
                  (property === 'opacity' ? 'linear' : self.mixer.config.animation.easing);
  
              return self.callFilters('ruleWriteTransitionRule', rule, arguments);
          }
docs: Combines the name of a CSS property with the appropriate duration and delay
  values to created a valid transition rule.
  
  @private
  @instance
  @since   3.0.0
  @param   {string}    property
  @param   {number}    staggerIndex
  @param   {number}    duration
  @return  {string}