BTW, my suggestion for testing `description_when_negated` on each matcher is to actually not call it directly from the test.  The behavior we're looking for is that `(~matcher).description` reads well (and having the matcher implement `description_when_negated` is just an implementation detail...plus, I suspect that some matchers won't need it as `Negation`'s fallback implementation of prefixing `not` will probably read fine on some matchers) -- so test `(~matcher).description`.