def _tokenize(text):
    """Return tokens from text.

    Text are splitted by tokens with space as seperator.
    All non alpha character discarded.
    All tokens are casefolded.
    """

    # split text by spaces and add only alpha character
    wordvoid = (''.join(ch for ch in w if ch.isalpha()) for w in text.split(' '))
    # discard empty element and lower others
    return (w.casefold() for w in wordvoid if w)