refactor: Add contracts
refactor: Add contracts

Bugfry/exercises,Bugfry/exercism,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercises,Bugfry/exercism,Bugfry/exercism,Bugfry/exercises,Bugfry/exercises