docs: function tweenProps (forPosition, currentState, originalState, targetState,
        duration, timestamp, easing) {
        var normalizedPosition = (forPosition - timestamp) / duration;
  
        var prop;
        for (prop in currentState) {
          if (currentState.hasOwnProperty(prop)) {
            currentState[prop] = tweenProp(originalState[prop],
              targetState[prop], formula[easing[prop]], normalizedPosition);
          }
        }
  
        return currentState;
      }