def snake_case(a_string):
partial = re.sub('<STR_LIT>', r'<STR_LIT>', a_string)<EOL>return re.sub('<STR_LIT>', r'<STR_LIT>', partial).lower()<EOL>
Returns a snake cased version of a string.

    :param a_string: any :class:`str` object.

    Usage:
        >>> snake_case('FooBar')
        "foo_bar"