Prolog脚本的意外输出

我希望

inHands.

返回如下内容：

leftHand(empty).
rightHand(empty).

inHands :-
    write("Left hand:"),
    nl,
    leftHand(X),
    tab(2),
    write(X),
    nl,
    nl,
    write("Right hand:"),
    rightHand(Y),
    tab(2),
    write(Y),
    nl.

 24 ?- inHands.
[76, 101, 102, 116, 32, 104, 97, 110, 100, 58]
  empty

[82, 105, 103, 104, 116, 32, 104, 97, 110, 100, 58]  empty
true.

然而，我看到的是：

Left hand:
  empty

Right hand:
  empty

---

我做错了什么？

结果我不得不用这样的单引号：

leftHand(empty).
rightHand(empty).

inHands :-
    write("Left hand:"),
    nl,
    leftHand(X),
    tab(2),
    write(X),
    nl,
    nl,
    write("Right hand:"),
    rightHand(Y),
    tab(2),
    write(Y),
    nl.

 24 ?- inHands.
[76, 101, 102, 116, 32, 104, 97, 110, 100, 58]
  empty

[82, 105, 103, 104, 116, 32, 104, 97, 110, 100, 58]  empty
true.