class T::Private::Methods::DeclBuilder

def implementation
  check_live!
  case decl.mode
  when Modes.abstract, Modes.override
    raise BuilderError.new("`.implementation` cannot be combined with `.#{decl.mode}`")
  when Modes.standard
    decl.mode = Modes.implementation
  when Modes.overridable
    decl.mode = Modes.overridable_implementation
  when Modes.implementation, Modes.overridable_implementation
    raise BuilderError.new(".implementation cannot be repeated in a single signature")
  end
  self
end