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

def rate(self):
        """
        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
        """
        if (
            (self.fragment_type != self.REGULAR) or
            (self.has_zero_length)
        ):
            return None
        return Decimal(self.chars / self.length)