new_contents: import abc
  
  
  class CubeABC(metaclass=abc.ABCMeta):
  
      @abc.abstractmethod
      def do_step(self, step):
          return NotImplemented
  
      @abc.abstractmethod
      def do_formula(self, formula):
          return NotImplemented
  
      def __call__(self, formula):
          return self.do_formula(formula)
  
subject: Create CubeABC