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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.
