code: public void addTransition(ElementConstraint constraint, ElementSelector<T> target) {
          buildState().addTransition(constraint, target.buildState());
      }
docs: Add an explicit transition based on an explicit constraints.
  <p>
  <b>TODO</b> This isn't quite consistent with the way I use
  constraints for other things.  The normal semantics for
  selectors is to say child(ElementConstraint...), ie a
  node selector + constraints on that selection.
  
  @param constraint
  @param target