code: func (r *Subresource) GetChange(key string) (interface{}, interface{}) {
  	new := r.data[key]
  	// No old data means no change,  so we use the new value as a placeholder.
  	old := r.data[key]
  	if r.olddata != nil {
  		old = r.olddata[key]
  	}
  	return old, new
  }
docs: // GetChange gets the old and new values for the value specified by key.