+traverse passed a test
=> push-r
rip
=> .(a `(tree item)`a)
+ ::
+ |- ^+ self
?: =(~ stack)
~& %rip-stack-gone
self
- ::
=^ direction self pop
?- direction
- %l ~& %rip-popped-l rip
- %r ~& %rip-popped-r
- => pop
- => ?: ?=([[%l *] *] stack)
- push-l
- push-r
+ %l ~& %rip-popped-l
+ => push-r
rip
+ ::
+ %r ~& %rip-popped-r
+ $
==
:: apply .f to a single node, updating .state, .stop, and .a
::
