code: public static <T> boolean isDescendantOf(TreeDef.Parented<T> treeDef, T child, T parent) {
  		T candidateParent = treeDef.parentOf(child);
  		while (candidateParent != null) {
  			if (candidateParent.equals(parent)) {
  				return true;
  			} else {
  				candidateParent = treeDef.parentOf(candidateParent);
  			}
  		}
  		return false;
  	}
docs: Returns true iff child is a descendant of parent.