> When (foreign-status-plist /foreign-status-object/) is handed something
> that's not a foreign-status-object, what should it do?  Would raising an
> non-continuable error always be the right thing??

"It is an error" to pass something other than a foreign-status object to
this procedure is the usual SRFI verbiage, meaning it's undefined
behavior. Or just don't mention it -- SRFI procedures do UB on undefined
argument types by default. Good to raise an exception, but not required.

What does `foreign-status-plist` do, return the whole plist? Does
enumerating the properties need to be in the API? If it does, then
`(foreign-status-properties st) -> list of symbols` would be better for
encapsulation. The implementor has more choice re: internal
representations and we can postpone the decision about transparent lambdas.