import unicodedata
import re


def ascify(s: str) -> str:
    """Normalize `s` to an ASCII form.

    The normalized form has only lowercase letters and spaces, suitable for use
    as search terms.
    """
    s = s.lower()
    s = unicodedata.normalize('NFKD', s)
    s = s.replace('æ', 'ae')
    s = s.replace('þ', 'th')
    s = s.replace('ð', 'th')
    s = re.sub('[^a-z ]', '', s)
    s = re.sub('[^a-z]+', ' ', s).strip()
    return s