'Gets the Gray code ordered subset of the specified rank.
Examples
>>> from sympy.combinatorics.subsets import Subset
>>> Subset.unrank_gray(4, [\'a\', \'b\', \'c\']).subset
[\'a\', \'b\']
>>> Subset.unrank_gray(0, [\'a\', \'b\', \'c\']).subset
See Also
iterate_graycode, rank_gray'

@classmethod
def unrank_gray(self, rank, superset):

    graycode_bitlist = GrayCode.unrank(len(superset), rank)
    return Subset.subset_from_bitlist(superset, graycode_bitlist)
