UWVisitor

==Responsibility== Generally, whoever creates the visitor should be responsible for closing it, i.e. not intermediate transform(v: Visitor) methods themselves.

==Self Closing== Given that common usage is most often single-valued (e.g. "{}"), rather than multi-valued (e.g. "{} {} {}"), Visitors may self-close (e.g. visitor.map{v => Try(v.close); v)} after a single value to prevent resource leaks, but are encouraged to expose both forms (i.e. single/multiple), if supportable.

==Multiple close() calls/Idempotency== Visitors are encouraged to respond gracefully if close() is called multiple times. If an underlying resource would throw if already closed, this may mean adding a private var isClosed: Boolean field to prevent multiple calls.