Audience asks how Schirp handles a mutation he decides not to kill because it looks equivalent. His reflex: sit back and ask why the mutation exists — Mutant's operators are laid out around explicit axioms (e.g. 'redundancy provides no value'), so an unkillable semantic-reduction mutation usually means the extra semantics were never needed; change the code to what the mutation showed. When the unkillable mutation actually stems from a weak spot in a called library (nil accepted where it shouldn't, malformed input silently swallowed), fix the library upstream and the mutation becomes dead. Truly equivalent cases (e.g. negative constant wrapped in abs) are rare; when they happen, a message-expectation test can pin behavior. Mutant has no built-in 'ignore this mutation' directive — deliberately, because ignoring usually means violating an axiom.