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

[benknoble]

https://github.com/drym-org/qi/blob/601d1b6d8515f5f61fe1d083d363b51f75e5a9af/qi-probe/probe.rkt#L67-L80

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.