declare local proc {ZeroExit N Is} case Is of I|Ir then if N+I\=0 then {ZeroExit N+I Ir} end end end in proc {NewThread P ?SubThread} Is Pt={NewPort Is} in proc {SubThread P} {Send Pt 1} thread {P} {Send Pt ~1} end end {SubThread P} {ZeroExit 0 Is} end end