def METHOD_NAME(self):
    """
    Remove sequences with a (too) high level of unknown tokens.
    """
    if "unk_token" not in self.params.special_tok_ids:
        return
    else:
        unk_token_id = self.params.special_tok_ids["unk_token"]
    init_size = len(self)
    unk_occs = np.array([np.count_nonzero(a == unk_token_id) for a in self.token_ids])
    indices = (unk_occs / self.lengths) < 0.5
    self.token_ids = self.token_ids[indices]
    self.lengths = self.lengths[indices]
    new_size = len(self)
    logger.info(f"Remove {init_size - new_size} sequences with a high level of unknown tokens (50%).")