Ruby mutation-testing tool authored by Markus Schirp. Takes code and its automated tests and applies a set of transformations (mutation operators) to discover unspecified semantics — code changes that no test catches. Core concepts: subjects (instance and class methods; potentially extensible to class-level DSL, constants, inheritance), match expressions (recursive-enumeration, class-scope, specific instance-/singleton-method forms) to tell the engine where to look, selection (tying subjects to tests via RSpec describe/context metadata or explicit coverage declarations in minitest), and mutation operators (semantic-reduction operators and auto-color replacements e.g. flipping < to <=, inverting &&/||, flipping integer signs). Produces unified-diff reports; alive mutations represent automatically-found holes equivalent to a human reviewer asking 'why can we do this change?'. Supports incremental mutation testing that restricts analysis to subjects touched in the current change, enabling CI use on large codebases. The 'aggregates' repository ships a mutation-testing script so contributors can run Mutant against any implementation; running Mutant against Krzywda's duck-typing implementation surfaced missing test coverage and structural duplication in a JIRA-inspired state machine.