def squash_flags(flags):
    """Remove lowercase flags if the respective uppercase flag exists

    >>> squash_flags('abc')
    'abc'
    >>> squash_flags('abcC')
    'ab'
    >>> squash_flags('CabcAd')
    'bd'
    """
    exclude = ''.join(f.upper() + f.lower() for f in flags if f == f.upper())
    return ''.join(f for f in flags if f not in exclude)