Returns a dependency extractor object
        @rtype: L{Cdependency_extractor}
        @return: a dependency extractor object

def get_dependency_extractor(self):
        """
        Returns a dependency extractor object
        @rtype: L{Cdependency_extractor}
        @return: a dependency extractor object
        """
        if self.dependency_layer is not None:    #otherwise there are no dependencies
            if self.my_dependency_extractor is None:
                self.my_dependency_extractor = Cdependency_extractor(self)
            return self.my_dependency_extractor
        else:
            return None