Principle to design subtypes that are safe to substitute instead of supertype, that is code intended to work with supertype keeps functioning as expected when handling objects of subtype.

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• tag solid
• Wikipedia article: Liskov substitution principle

^{Substitutability is a principle in object-oriented programming. It states that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.). More formally, the Liskov substitution principle (LSP) is a particular definition of a subtyping relation, called (strong) behavioral subtyping...}