new_contents: from zope.interface import implements
  
  from IOperation import IOperation
  
  
  class Plus(object):
      implements(IOperation)
  
      def __call__(self, a, b):
          return a + b
  
  
  class Minus(object):
      implements(IOperation)
  
      def __call__(self, a, b):
          return a - b
  
  
  ### alternative way to make utility component (using not class-adviser on class level -> using function classImplements)
  # from zope.interface import classImplements
  # classImplements(Host, IHost)
  ### also in Python 2.6 and later you can use class decorator @implementer(IFoo)
  
subject: Create utils.py