ok, i think i've got a handle on the continuations/coroutine/closures concept.
reading http://mojave.caltech.edu/papers/cont-tut.ps it would seem
actually quite easy (to the point where i'm surprised so many people
can explain it so poorly that quite a brouhaha exists around it).

but now, the issue seems low-level.  does the ruby interpreter bother
using a stack?  (my guess would be no, given garbage collection and
built-in closures.)
is 'return' simply an alias for "calling the caller"?  (and thus
'yield' an alias for calling the anonymous (lambda) closure passed in
as a "special" (syntactally) argument?)

peace,
-z