Default kind of tree rep - :vec, :list, or :map

Return the type of the tree location (zipper branch) loc.  This
will be one of the extent zipper types (from zip/zipper) available.
In practice, it is going to be a vector, seq, or map.

Return whether x is a valid tree location (aka zipper branch)

Predicate defining what constitutes a map branch for map zippers

Define a map zipper.  Returns a zipper for nested maps, given a
starting/root map

Return an empty location of type kind an element of
#{:vec :map :list}