code: def _insert(self, key, records, count):
          
          if key not in self.records:
              assert key not in self.counts
              self.records[key] = records
              self.counts[key] = count
          else:
              self.records[key] = np.vstack((self.records[key], records))
              assert key in self.counts
              self.counts[key] += count
docs: Insert records according to key