[AstralStorm]

Write automated proofs? Probably best but tooling is junk - it's rarely supported.

Constraint or property based tests as opposed to value tests? (That depends on how unique.) Usually the only thing that does unique things is a direct mapping, everything else is either an equation or has other preconditions, postconditions and internal or external properties. It also makes the logic more directly visible and refactoring easier presuming tests are reasonably written.

Ensure coverage with a tool to be sure.