'Generates the previous Gray code ordered subset.
Examples
>>> from sympy.combinatorics.subsets import Subset
>>> a = Subset([2, 3, 4], [1, 2, 3, 4, 5])
>>> a.prev_gray().subset
[2, 3, 4, 5]
See Also
iterate_graycode, next_gray'

def prev_gray(self):

    return self.iterate_graycode((-1))
