old_contents: __author__ = 'amelie'
  
  
  class InvalidNGramError(ValueError):
      def __init__(self, n, n_gram):
          self.n = n
          self.n_gram = n_gram
  
      def __str__(self):
          error_message = '{} is not a possible {:d}_gram for this alphabet'.format(self.n_gram, self.n)
          return error_message
  
  
  class InvalidNError(ValueError):
      def __init__(self, n):
          self.n = n
  
      def __str__(self):
          error_message = 'n must be greater than zero. Got: n={:d}'.format(self.n)
          return error_message
new_contents: __author__ = 'amelie'
  
  
  class InvalidNGramError(ValueError):
      def __init__(self, n, n_gram):
          self.n = n
          self.n_gram = n_gram
  
      def __str__(self):
          error_message = "{} is not a possible {:d}_gram for this alphabet".format(self.n_gram, self.n)
          return error_message
  
  
  
  class InvalidNError(ValueError):
      def __init__(self, n):
          self.n = n
  
      def __str__(self):
          error_message = 'n must be greater than zero. Got: n={:d}'.format(self.n)
          return error_message
subject: add exceptions