old_contents: import re
  
  
  def is_valid_tn(self, tn_string):
      valid_tn = re.compile('^\d{3}-\d{3}-\d{4}$')
      return re.match(valid_tn, tn_string)
  
  
new_contents: import re
  
  
  def is_valid_tn(tn_string):
      valid_tn = re.compile('^\d{3}-\d{3}-\d{4}$')
      return re.match(valid_tn, tn_string)
  
subject: remove requirement for self from valid phone number validator