Focus

Focuses are lists of numbers where longer lists are treated as focuses on their parent lists. As such, `Focus []` is considered unfocused (it is EQ to all focuses), `Focus [1, 1]` is EQ to `Focus [1, 1, 3]` because their roots are equal, and so on. This isn't useful for testing true equality, but it is quite useful for traversing and pruning indexed rose trees from one locus to another.