code: public RollingUpdateOp yield() {
      // Do nothing
      return new RollingUpdateOp(ImmutableList.<ZooKeeperOperation>of(),
          ImmutableList.<Map<String, Object>>of());
    }
docs: Don't advance to the next task -- yield and have the current task be executed again in the
  next iteration.
  
  @return {@link RollingUpdateOp}