Silence warning with type cast.
Some string generating methods for debugging.
Convenience for walking the sparse array tree. Useful? I don't know yet. Maybe not.
-updateRange:set: looked broken. This makes it better, but it still needs proper tests to be written.
Working on finishing implementation of the BABitArray protocol, but running into some difficulties.

Saving my current place before I rip it out and lose my history.
Test the tree walking support.
Test walking nodes in a 2D sparse bit array.

Deserves a general case.
Starting work on more thorough testing for range and rectangle reading and writing.

The string reading relies on the former.
Support for generating 2D string representation of sparse array (for testing).
Move more methods to protocol, and implement as required.
Add 3D size conveniences.
Add another string-generating convenience.
Add required bit so previously added method will work.
Fix bug in recursion.
Move methods to protocol-implementing category since moving method declarations to protocol.
Fix array writing to support sparse bit array. Slow implementation for now. Add unit test.
Even if we don't return the offset to the caller, recursive invocations require it.