class IntervalSkipList

def overlapping(range)
  markers, first_node = containing_with_node(range.first)
  cur_node = first_node
  begin
    markers.concat(cur_node.forward_markers.flatten)
    cur_node = cur_node.forward[0]
  end while cur_node.key < range.last
  return markers.uniq, cur_node
end