code: function(anew, old, className) {
        if (old) {
          old.classList.remove(className);
        }
        if (anew) {
          anew.classList.add(className);
        }
      }
docs: Remove class from old, add class to anew, if they exist.
  
  @param classFollows
  @param anew A node.
  @param old A node
  @param className