@mkobetic I don't love the naming. I don't feel like it fits well with the stdlib naming. Can we borrow from [`runtime.MemStats`](http://golang.org/pkg/runtime/#MemStats) and use:

```go
BranchAlloc  // total bytes used for branch pages.
BranchInuse  // total bytes used for branch data.
LeafAlloc    // total bytes used for leaf pages.
LeafInuse    // total bytes used for leaf data.
```

And then we can calculate free bytes by `Alloc - Inuse`. What do you think?