code: @Override
  	public void set(T result) {
  		assert !isComplete();
  		this.result = result;
  		this.exception = null;
  		complete(result);
  	}
docs: Sets the result of this {@code SettablePromise} and
  completes it. {@code AssertionError} is thrown when you
  try to set result for an already completed {@code Promise}.