code: def add(self, chrom, element):
          
  
          self._trees.setdefault(chrom, IntervalTree()).insert_interval( element )
docs: insert an element. use this method as the IntervalTree one.
          this will simply call the IntervalTree.add method on the right tree
  
          :param chrom: chromosome
          :param element: the argument of IntervalTree.insert_interval
          :return: None