A transformation that takes a concrete subject into a different form such that, if undetected by the tests, forms a reported mutation. Two classes discussed: semantic-reduction operators (remove semantics, e.g. dropping a method call from a method body) and auto-color replacements (change one operator for another, e.g. < to <=, && to ||, positive integer to negative). Mutant's operator set is laid out alongside explicit axioms (notably 'redundancy provides no value'), which constrains operators so that equivalent mutations remain rare.