Sam

package P is

   Shared : Natural := 0;

   procedure Maybe_Increment;

end P;

package body P is

   protected Lock is
      entry Maybe_Lock (Lock_Acquired : out Boolean);
      entry Maybe_Unlock (Was_Locked : Boolean);
   private
      Is_Locked : Boolean := False;
   end Lock;

   protected body Lock is

      entry Maybe_Lock (Lock_Acquired : out Boolean) when True is
      begin
         Lock_Acquired := not Is_Locked;
         Is_Locked     := True;
      end Maybe_Lock;

      entry Maybe_Unlock (Was_Locked : Boolean) when True is
      begin
         if Was_Locked then
           Is_Locked := False;
         end if;
      end Maybe_Unlock;

   end Lock;

   procedure Maybe_Increment is
      L : Boolean;
   begin
      Lock.Maybe_Lock (L);
      if L then
         Shared := Shared + 1;
      end if;
      Lock.Maybe_Unlock (L);
   end Maybe_Increment;

end P;