Concurrency in Scheme

[VERACSD@A.ISI.EDU]

I am in the process of writing a metacircular Lisp interpreter which
accommodates concurrent programming constructs (e.g. parbegin/parend's
and semaphores) and have recently come across *engines* in PC Scheme.

Engines seem like they offer much potential for this sort of thing,
however, they seem somewhat tricky.  It is especially unclear to me 
how to elegantly handle waits & signals for semaphores.  
(There must be a better way than Ben-Ari's Pascal concurreny simulator.)

Any advice, pointers to sources, or code would be much appreciated.

Cris Kobryn
VERACSD.CK@A.ISI.EDU