import unicodedata

def derp(a_string):
    """
    Get rid of all non-letters, make uppercase letters lowercase
    """
    new_string = ''
    for char in a_string:
        if unicodedata.category(char) == 'Lu':
            new_string += char.lower()
        elif unicodedata.category(char) == 'Ll' or unicodedata.category(char) == 'Lo':
            new_string += char

    return new_string