code: def rate(self):
          
          if (
              (self.fragment_type != self.REGULAR) or
              (self.has_zero_length)
          ):
              return None
          return Decimal(self.chars / self.length)
docs: The rate, in characters/second, of this fragment.
  
          If the fragment is not ``REGULAR`` or its duration is zero,
          return ``None``.
  
          :rtype: ``None`` or :class:`~aeneas.exacttiming.Decimal`
  
          .. versionadded:: 1.2.0