This module collection manages continuations in general, most often in the
form of cooperative threads (also called coros, or simply "coro" in the
documentation). They are similar to kernel threads but don't (in general) run
in parallel at the same time even on SMP machines. The specific flavor of
thread offered by this module also guarantees you that it will not switch
between threads unless necessary, at easily-identified points in your
program, so locking and parallel access are rarely an issue, making thread
programming much safer and easier than using other thread models.