if (tail.NE(Dequeue.TAIL_INITIAL_VALUE)) {
if (tail.NE(Deque.TAIL_INITIAL_VALUE)) {
  private final void insertOverflow(int arity) {    if (VM_Interface.VerifyAssertions) VM_Interface._assert(arity == queue.getArity());    if (tail.NE(Dequeue.TAIL_INITIAL_VALUE)) {      closeAndEnqueueTail(arity);    }    tail = queue.alloc().add(bufferLastOffset(arity)).add(BYTES_IN_ADDRESS);    Plan.checkForAsyncCollection(); // possible side-effect of alloc()  }