SpatialPrefixTree makes "Node"s which are basically a rectangular spatial region that is more colloquially referred to as a "Cell". It was named "Cell" in the first place and for whatever reason, Ryan and/or Chris renamed it as part of extracting it to a top level class from an inner class. Most comments and variable names still use the "cell" terminology. I'm working on an algorithm that keeps track of a tree of "nodes" and it has gotten confusing which kind of node I'm referring to, as each Node has one cell.
In maybe a week or so if there isn't discussion to the contrary, I'm going to commit a rename it back to "Cell". And... while we're on this naming subject, perhaps "SpatialPrefixTree" could be named "SpatialGrid" ? FWIW the variables referring to it are always "grid".