Permute all casings of a given string.

    A pretty algorithm, via @Amber
    http://stackoverflow.com/questions/6792803/finding-all-possible-case-permutations-in-python

def all_casings(input_string):
    """
    Permute all casings of a given string.

    A pretty algorithm, via @Amber
    http://stackoverflow.com/questions/6792803/finding-all-possible-case-permutations-in-python
    """
    if not input_string:
        yield ""
    else:
        first = input_string[:1]
        if first.lower() == first.upper():
            for sub_casing in all_casings(input_string[1:]):
                yield first + sub_casing
        else:
            for sub_casing in all_casings(input_string[1:]):
                yield first.lower() + sub_casing
                yield first.upper() + sub_casing