User level (aka green) threading is difficult. Very few implementations work (Erlang, Golang, <any others>+) and it takes a great deal of time and effort to get

User level (aka green) threading is difficult. Very few implementations work (Erlang, Golang, <any others>+) and it takes a great deal of time and effort to get to that "fully working" state. +Perhaps the new Java fiber stuff works but I don't have enough data to be sure yet.