#     var dquadtree = new DQuadTree[Point[Int]](2)
	#     var p1 = new Point[Int](0,0)
	#     var p2 = new Point[Int](0,9)
	#     var p3 = new Point[Int](9,0)
	#     dquadtree.add(p1)
	#     dquadtree.add(p2)
	#     dquadtree.add(p3)
	#     var result = dquadtree.items_overlapping(p3)
	#     assert result.length == 1
	#     result.clear
	#     var p4 = new Point[Int](9,9)
	#     result = dquadtree.items_overlapping(p4)
	#     assert result.length == 0
	#     result = dquadtree.items_overlapping(p4.padded(10))
	#     assert result.length == 3
	private fun items_overlapping_in(region: Boxed[Numeric], mdata: SimpleCollection[E])
	do
		if self.is_leaf and data.length >= item_limit then
			subdivide
			var data_copy = data
			data = new Array[E]
			#add to the right Node
			for d in data_copy do
				add_to_children(d)
			end
		end
		for i in data do if i.intersects(region) then mdata.add(i)

		if children.not_empty then
			var center = center
			assert center != null
			if center.x > region.right then
				if center.y > region.top then
					children[0].items_overlapping_in(region, mdata)
				else if center.y < region.bottom then
					children[1].items_overlapping_in(region, mdata)
				else
					children[0].items_overlapping_in(region,mdata)
					children[1].items_overlapping_in(region, mdata)
				end
			else if center.x < region.left then
				if center.y > region.top then
					children[3].items_overlapping_in(region, mdata)
				else if center.y < region.bottom then
					children[2].items_overlapping_in(region, mdata)
				else
					children[3].items_overlapping_in(region, mdata)
					children[2].items_overlapping_in(region, mdata)
				end
			else if center.y > region.top then
				children[0].items_overlapping_in(region, mdata)
				children[3].items_overlapping_in(region, mdata)
			else if center.y < region.bottom then
				children[1].items_overlapping_in(region, mdata)
				children[2].items_overlapping_in(region, mdata)
			else
				children[0].items_overlapping_in(region, mdata)
				children[1].items_overlapping_in(region, mdata)
				children[2].items_overlapping_in(region, mdata)
				children[3].items_overlapping_in(region, mdata)
			end
		end
	end