[plt-scheme] "Standard" coroutine library suitable for building filters as well as generators

I think the term is "fully asymmetric" coroutines, but please correct
me if I'm wrong.  I'm not look for just Python style generators, but
a much more robust coroutine implementation.

I'm looking for a fairly "canonical, full" coroutine implementation,
where a "yield" like operator can work at activation levels other
than just the top level, and where a "yield" style operator can
function as both a LHS or RHS operator (or two separate operators),
facilitating sending values to a coroutine in a "filter chain" style
structured program.

Googling, I find bits here and there about coroutines in PLT Scheme,
but no pointers to a "definitive" or "standard" implementation.

If there are runtime-system issues that make this difficult to
implement in a Scheme like PLT, I'd love to understand this better
as well.

Many thanks!

Scott