quadtree.visitAfter(callback)
Visits each node in the quadtree in post-order traversal, invoking the specified callback with arguments node, x0, y0, x1, y1 for each node, where node is the node being visited, ⟨x0, y0⟩ are the lower bounds of the node, and ⟨x1, y1⟩ are the upper bounds, and returns the quadtree. (Assuming that positive x is right and positive y is down, as is typically the case in Canvas and SVG, ⟨x0, y0⟩ is the top-left corner and ⟨x1, y1⟩ is the lower-right corner; however,