'Returns the currently referenced Gray code as a bit string.
Examples
>>> from sympy.combinatorics.graycode import GrayCode
>>> GrayCode(3, start=\'100\').current
\'100\''

@property
def current(self):

    rv = (self._current or '0')
    if (type(rv) is not str):
        rv = bin(rv)[2:]
    return rv.rjust(self.n, '0')
