@classmethod
def _prime_numbers_until(cls, limit):
    """This is totally a funny test method."""
    res = cls[2:limit]
    for i in range(2, limit // 2 + 1):
        if i in res:
            aux = i + i
            while aux < limit:
                if aux in res:
                    res.remove(aux)
                aux += i
    return res
