code: 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;
      }
docs: /*!
  Calculates the interpolated tween values of an Object for a given
  timestamp.
  @param {Number} forPosition The position to compute the state for.
  @param {Object} currentState Current state properties.
  @param {Object} originalState: The original state properties the Object is
  tweening from.
  @param {Object} targetState: The destination state properties the Object
  is tweening to.
  @param {number} duration: The length of the tween in milliseconds.
  @param {number} timestamp: The UNIX epoch time at which the tween began.
  @param {Object} easing: This Object's keys must correspond to the keys in
  targetState.