`STOP` is not cooperative.

[python-processing-unit]

According to the specification section 9.1.9:

THR: STOP(THR: thread) = MUST cooperatively stop thread, mark it finished, and return the resulting handle.

In the implementation, however, STOP only marks the handle as finished. The worker keeps running, so a supposedly stopped thread can still continue executing, and AWAIT can block indefinitely on it.

Prefix REPL. Enter statements, blank line to run buffer.
>>> THR(t){ WHILE(0d1){} }
>>> STOP(t)
>>> AWAIT(t)

AWAIT does not return here, as the worker was never actually stopped.