Ch5, Ex4

PartI- Well, One way to understand it is by seeing that the assertion, which said "The sum of elements on the port's stream is greater than or equal to the number of active threads." ,that was stated to prove the correctness of NewThread symantics is no longer always true with this new definition of SubThread. Let us look carefully at the two definitions of SubThread and the embedded comments that provide the explaination.

% Correct Version
proc {SubThread P}
   %Notice that +1 is sent to the Port's stream before
   %starting the new thread, this makes sure that sum of
   %elements is *incremented before* any new thread is started
   {Send Pt 1} 
   thread
      {P} {Send Pt  ~1}
   end
end
% Incorrect Version
proc {SubThread P}
   %thread is started before sending +1 to the port's stream
   %so right after creation of this thread, if scheduler
   %doesn't execute it then a thread is created but sum of
   %elements is not incremented and that means assertion is
   %no longer true.
   thread
      {Send Pt 1}
      {P} {Send Pt  ~1}
   end
end 

PartII- Required Example

declare NewThread
local
   proc {ZeroExit N Is}
      case Is of I|Ir then
         if N+I\=0 then {ZeroExit N+I Ir} 
         else
            % 'ZeroExit Finished' is displayed when call to
            % ZeroExit is over.
            {Browse 'ZeroExit Finished'}
         end
      end
   end
in
   proc {NewThread P ?SubThread}
      Is Pt={NewPort Is}
   in
      proc {SubThread P}
         thread
            {Send Pt 1}
            {P} {Send Pt  ~1}
         end
      end
      {SubThread P}
      {ZeroExit 0 Is}
   end
end

local SubThread in
   {NewThread
    proc {$}
       {Browse 'T1 started'}
       {SubThread
        proc {$}
           {Browse 'T2 started'} {Delay 2000}
           {Browse 'T2 finished'}
        end} %T2
       {Browse 'T1 finished'}
    end
    SubThread}
   {Browse 'NewThread Finished'}
end 

Notice that "ZeroExit Finished" is printed before "T2 finished" that is sum of elemtents on the port stream became zero though T2 was not finished.