(tick leaf-node stash)
=> Result w/ leaf-node' and stash'

(tick! leaf-node)
=> Result w/ leaf-node'

(part composite-node)
=> child-node

(join composite-node Result w/ child-node' and optional stash')
=> Result w/ composite-node' and optional stash'

Behavior Tree basic building blocks.

wikla.core aims to follow terminology outlined in _Towards a Unified Behavior
Trees Framework for Robot Control_ by Marzinotto et al. (2014), especially
sections I-IV.

Whenever a tick is done it yields with a Result. That Result is tied to Status
flag carried by the node and contains the modified state. When root returns
a Result -- the tick is done.

Leaf nodes get ticked:

    (tick leaf-node stash)
    => Result w/ leaf-node' and stash'

    (tick! leaf-node)
    => Result w/ leaf-node'

Composite nodes provide children for execution via part call:

    (part composite-node)
    => child-node

And compute Result when their child comes back carried by Result:

    (join composite-node Result w/ child-node' and optional stash')
    => Result w/ composite-node' and optional stash'

All this is pure by default, using only immutable data.

For completeness a fourth status is introduced: Ready. It's the default status
for all freshly created nodes that were not yet ticked and can be used to ask
the Ticker for immediate re-execution during the tick. Unlike Success, Failure
or Running it will not be reported outside of the Ticker. The source paper
calls it NotTicked and only uses it for initial state.

Nodes into DOTs.

Even more nodes.

Tools that make your life easier while working with wikla.

Pre-cooked nodes & convenience constructors.

Zippers over nodes!

One and only thing you need to turn built-in Nodes into zippers is the
`zipper` function provided by this namespace. To allow custom nodes implement
Zip protocol accordingly.

Protip: zip-make-node expects children to be in shape of zip-children result.