old_contents: #!/usr/bin/env python3
  """Classes and methods for working with all Turing machines."""
  
  import abc
  
  import automata.base.exceptions as exceptions
  import automata.tm.exceptions as tmexceptions
  from automata.base.automaton import Automaton
  
  
  class TM(Automaton, metaclass=abc.ABCMeta):
      """An abstract base class for Turing machines."""
  
      def _validate_input_symbol_subset(self):
          if not (self.input_symbols < self.tape_symbols):
              raise exceptions.MissingSymbolError(
                  'The set of tape symbols is missing symbols from the input '
                  'symbol set ({})'.format(
                      self.tape_symbols - self.input_symbols))
  
      def _validate_nonfinal_initial_state(self):
          """Raise an error if the initial state is a final state."""
          if self.initial_state in self.final_states:
              raise exceptions.InitialStateError(
                  'initial state {} cannot be a final state'.format(
                      self.initial_state))
  
new_contents: #!/usr/bin/env python3
  """Classes and methods for working with all Turing machines."""
  
  import abc
  
  import automata.base.exceptions as exceptions
  from automata.base.automaton import Automaton
  
  
  class TM(Automaton, metaclass=abc.ABCMeta):
      """An abstract base class for Turing machines."""
  
      def _validate_input_symbol_subset(self):
          if not (self.input_symbols < self.tape_symbols):
              raise exceptions.MissingSymbolError(
                  'The set of tape symbols is missing symbols from the input '
                  'symbol set ({})'.format(
                      self.tape_symbols - self.input_symbols))
  
      def _validate_nonfinal_initial_state(self):
          """Raise an error if the initial state is a final state."""
          if self.initial_state in self.final_states:
              raise exceptions.InitialStateError(
                  'initial state {} cannot be a final state'.format(
                      self.initial_state))
  
subject: Remove unused tm.exceptions import