def tokens(text, tok_size=3):
    """ Returns a list of all substrings contained in 'text' of size 'tok_size'

    :param text: (string) text to tokenize
    :param tok_size: length of substrings
    :return: (list) tokens of 'text'
    """
    return [text[i : i + tok_size] for i in range(len(text) - tok_size + 1)]