'Gets the rank of a partition.
Examples
>>> from sympy.combinatorics.partitions import Partition
>>> a = Partition([1, 2], [3], [4, 5])
>>> a.rank
13'

@property
def rank(self):

    if (self._rank is not None):
        return self._rank
    self._rank = RGS_rank(self.RGS)
    return self._rank
