The latter refers to the before and after thunks of dynamic-wind,

while the former refers only to parameter bindings.  I believe you

intend for promises to be run in the dynamic extent of the time

that they were created, not the environment.

Have you run any benchmarks for this?  It seems an extremely

heavyweight operation for something which should be very

lightweight.  The dependence on dynamic-wind also means

performance will be highly variable per implementation.

Note I think delay-force/lazy were a mistake.  Their semantics

are of course desired, but I think in retrospect it's sufficient to

require (delay (force x)) be statically detected and the same

guarantees made - this can be done portably.

-- 

Alex