old_contents: #!/usr/bin/env python
  """
  :Author Patrik Valkovic
  :Created 23.06.2017 16:39
  :Licence GNUv3
  Part of grammpy
  
  """
  
  from .Rule import Rule
  
  class IsMethodsRuleExtension(Rule):
  
      @classmethod
      def is_regular(cls):
          return False
  
      @classmethod
      def is_contextfree(cls):
          return False
  
      @classmethod
      def is_context(cls):
          return False
  
      @classmethod
      def is_unrestricted(cls):
          return False
new_contents: #!/usr/bin/env python
  """
  :Author Patrik Valkovic
  :Created 23.06.2017 16:39
  :Licence GNUv3
  Part of grammpy
  
  """
  
  from .Rule import Rule
  
  
  class IsMethodsRuleExtension(Rule):
      @classmethod
      def is_regular(cls):
          return False
  
      @classmethod
      def is_contextfree(cls):
          return False
  
      @classmethod
      def is_context(cls):
          return False
  
      @classmethod
      def is_unrestricted(cls):
          return False
  
      @classmethod
      def is_valid(cls):
          return False
  
subject: Add header of Rule.isValid method