This definition is elegant and easy to work with abstractly (such as when proving theorems about properties of trees), as it expresses a tree in simple terms: a list of one type, and a pair of two types.
这种定义是精致的，易于工作的抽象性（如用于证明关于树的属性的定理），因为它用简单术语表示一棵树：一个类型的列表，两个类型组成的一对。