Mutant-specific concept telling the engine where to look for subjects. Three forms: (1) recursive enumeration over a parent namespace, descending into all subjects automatically; (2) class scope, working on instance and singleton methods within a class; (3) a specific instance method or a specific singleton method. Essential because a project may have 100 dependencies and 100k lines of code — without match expressions the engine would be unusable. A good top-level namespace makes the recursive form trivial; for flatter layouts a small wrapper can enumerate classes from disk and pass them on the command line.