Usually an invariant is something you explicitly state about a data structure or algorithm, and which must hold before and after an operation, but can be tempor

Usually an invariant is something you explicitly state about a data structure or algorithm, and which must hold before and after an operation, but can be temporarily violated. What you are talking about here are assumptions, which are usually implicit and sometimes not even part of the thought process. One purpose of writing a design document is considering and stating the relevant assumptions.