Should probe's implementation use parameterize? Where? What are the semantics of possibly-nested probe/readout?

[benknoble]

qi/qi-probe/probe.rkt

Lines 67 to 80 in 601d1b6

	(define-syntax-parse-rule (probe flo)
	(call/cc
	(λ (return)
	(source return) ; save the continuation to the `source` parameter
	flo)))
	(define-qi-syntax-parser readout
	[_:id #'(esc
	(λ args
	(let ([src (source)])
	;; reset source to avoid the possibility of stale
	;; continuations used later outside of a `probe`
	(source default-source)
	(apply src args))))])

It might obviate the need for (source default-source), but does the parameterize belong around call/cc or within it? What happens in the presence of a threading (as in thread) backend? Etc.