code: final boolean unlink(Index<K,V> succ) {
              return node.value != null && casRight(succ, succ.right);
          }
docs: Tries to CAS right field to skip over apparent successor
  succ.  Fails (forcing a retraversal by caller) if this node
  is known to be deleted.
  @param succ the expected current successor
  @return true if successful