def dragon_step(a):
    """
    Call the data you have at this point "a".
    Make a copy of "a"; call this copy "b".
    Reverse the order of the characters in "b".
    In "b", replace all instances of 0 with 1 and all 1s with 0.
    The resulting data is "a", then a single 0, then "b".
    """
    b = a[::-1]
    b = ''.join(['0' if c == '1' else '1' for c in b])
    return a + '0' + b