declare
fun {NewLock}
   Token={NewCell unit}
   CurThr={NewCell unit}
   proc {Lock P}
      if {Thread.this}==@CurThr then
         {P}
      else Old New in
         {Exchange Token Old New}
         {Wait Old}
         CurThr:={Thread.this}
         try {P} finally
            CurThr:=unit
            New=unit
         end
      end
   end
in
   'lock'('lock':Lock)
end