Hi, thank you for your library!

I've started using it to store terrain (rectangles) for an OpenGL project. fmap/tmap'ing over the QuadTree has worked very well so far.

However, I would now like to cull some of the terrain using the camera's position and angle. I thought I might use filterTree to create a new, slimmer QuadTree, but it seems to return a list.

I have two questions / issues:

1. Does filterTree explore a node's children if the node itself fails the filter test?
2. Is there a reason behind returning a list instead of a new QuadTree?

Thanks!

Hello! Are you open to a small pull request contribution along the lines of the following?

-- |Unsafe sanity test lens that makes sure a given location index exists
-- within the relevant 'QuadTree'.
verifyLocation :: HCS => Location -> Lens' (QuadTree a) (QuadTree a)
verifyLocation index f qt
  | index `outOfBounds` qt = error $ ""
    ++ "Location index out of QuadTree bounds."
    ++ " Attempted index: " ++ show index ++ ","
    ++ " Max index: " ++ show (treeLength qt - 1, treeWidth qt - 1)
  | otherwise              = f qt

type HCS = HasCallStack

If so I intend to clean up and submit one.