Delimited continuations are in the Racket standard library. Racket's thread system isn't built directly on them but is also lightweight. The in-progress work to

Delimited continuations are in the Racket standard library. Racket's thread system isn't built directly on them but is also lightweight. The in-progress work to run Racket in Chez Scheme uses an implementation more like what wingo describes, based on what's called "engines".