里氏代换原则

若对于每一个类型S的对象o1，都存在一个类型T的对象o2，使得在所有针对T编写的程序P中，用o1替换o2后，程序P的行为功能不变，则S是T的子类型。

What is wanted here is something like the following
substitution property: If for each object o1 of type S there is an
object o2 of type T such that for all programs P defined in terms of T,
the behavior of P is unchanged when o1 is substituted for
o2 then S is a subtype of T.