def unicode_sorter(input):
    """ This function implements sort keys for the german language according to
    DIN 5007."""
    key1 = input.lower()
    key1 = key1.replace(u'ä', u'a')
    key1 = key1.replace(u'ö', u'o')
    key1 = key1.replace(u'ü', u'u')
    key1 = key1.replace(u'ß', u'ss')
    return key1
