我一直试图了解在Prolog中如何评估以下查询：
reflection(leaf,leaf).
reflection(node(S1,S2),node(T2,T1)) :-
reflection(S1,T1),
reflection(S2,T2).
symmetric(T) :-
reflection(T,T).
| ?- symmetric(X).
X = leaf
X = node(leaf,leaf)
X = node(node(leaf,leaf),node(leaf,leaf))
X = node(node(leaf,node(leaf,leaf)),node(node(leaf,leaf),leaf))
X = ...
我把它实现到X被实例化为leaf然后到node（leaf，leaf），但我无法理解后者。请解释一下，谢谢！