def _get_neg_skip(self, mol, pos_count):
        """Get negatively charged atoms to skip (up to pos_count)."""
        neg_skip = set()
        if pos_count:
            # Get negative oxygens in charge-separated nitro groups TODO: Any other special cases to skip?
            for occurrence in mol.GetSubstructMatches(self.nitro):
                neg_skip.add(occurrence[-1])
                if len(neg_skip) >= pos_count:
                    return neg_skip
            # Get strongest ionized acids
            for position, pair in enumerate(self.acid_base_pairs):
                for occurrence in mol.GetSubstructMatches(pair.base):
                    neg_skip.add(occurrence[-1])
                    if len(neg_skip) >= pos_count:
                        return neg_skip
        return neg_skip
Get negatively charged atoms to skip (up to pos_count).