Comment by BoingBoomTschak

Just a small "ackchyually": Go is basically a modern Limbo which is itself based on Alef and there was an official "Alef for C" thing in Plan 9 in libthread (https://9p.io/magic/man2html/2/thread)

EDIT: looks like it was ported on UNIX as part of Plan9Port (https://github.com/9fans/plan9port/blob/master/src/libthread...)