code: def __get_keys(self, chrom, start, end=None):
      
      keys = []
      if chrom in self.whole_chrom_files:
        keys.append(self.whole_chrom_files[chrom])
      if chrom in self.partial_trees:
        if end is not None:
          hits = self.partial_trees[chrom].intersectingInterval(start, end)
        else:
          hits = self.partial_trees[chrom].intersectingPoint(start, end)
        for hit in hits:
          keys.append(hit.key)
      return keys
docs: Get the hash keys in whole/partial_chrom_files that overlap the interval.
  
      :return: list of hash keys.