old_contents: class MonoidLawTester:
  
      def __init__(self, monoid, value):
          self.monoid = monoid
          self.value = value
  
      def left_identity_test(self):
          monoid = self.monoid(self.value)
          assert monoid.concat(monoid.neutral()) == monoid
  
      def right_identity_test(self):
          monoid = self.monoid(self.value)
          assert monoid.neutral().concat(monoid) == monoid
  
      def test(self):
          self.left_identity_test()
          self.right_identity_test()
  
  
new_contents: class MonoidLawTester:
  
      def __init__(self, monoid, value):
          self.monoid = monoid
          self.value = value
  
      def left_identity_test(self):
          monoid = self.monoid(self.value)
          assert monoid.concat(monoid.neutral()) == monoid
  
      def right_identity_test(self):
          monoid = self.monoid(self.value)
          assert monoid.neutral().concat(monoid) == monoid
  
      def test(self):
          self.left_identity_test()
          self.right_identity_test()
  
subject: fix flake8