Some of these cooperatively multithreaded implementations have "green" varieties of all your standard functions; these are greenlet-aware (that is, aware of the cooperative threading & I/O loop that's happening) functions that do things like, for example, sleep. So, you might have a my_green_library.sleep and a calls_the_os.sleep; the latter of which will yield the hardware thread directly to the OS, and block that thread completely until its done. Whereas the former will perform a sort of userland context switch, and note something to the I/O loop, and then sleep until the next event.

Worse, this problem makes composition hard: you need to know the entire implementation of any function you call, in order to be aware of whether or not it will cause the calling thread to block.