'Return the dictionary mapping r to Nr, the number of samples with frequency r, where Nr > 0.
:type bins: int
:param bins: The number of possible sample outcomes.  ``bins``
is used to calculate Nr(0).  In particular, Nr(0) is
``bins-self.B()``.  If ``bins`` is not specified, it
defaults to ``self.B()`` (so Nr(0) will be 0).
:rtype: int'

def r_Nr(self, bins=None):

    _r_Nr = defaultdict(int)
    for count in self.values():
        _r_Nr[count] += 1
    _r_Nr[0] = ((bins - self.B()) if (bins is not None) else 0)
    return _r_Nr
