The process Mutant uses to find the corresponding tests for a given subject. Selection defines mutation-testing speed — running all tests against every mutation would be prohibitively slow. RSpec's describe/context/nested primitives act as an implicit selection criterion; honest describe blocks are therefore critical (too-broad descriptions select far too many tests — terrible runtime; too-narrow ones select far too few — tabular coverage). minitest has too little implicit metadata, so Mutant requires explicit coverage declarations stating which test class covers which expression.