server(Free, Allocated) ->
   receive
      {From, alloc} ->
         allocate(Free, Allocated, From);
      {From, {free, R}} ->
         free(Free, Allocated, From, R);
 
      {'EXIT', From, _} ->
         check(Free, Alloctaed, From);
   end.

allocate([R|Free], Allocated, From) ->
   link(From), %%% Causes 'EXIT' messages to be sent %%%
   From ! {resource_alloc, {yes, R}},
   server(Free, [{R,From}|Allocated]);
allocate([], Allocated, From) ->
   From ! {resource_alloc, no},
   server([], Allocated).

free(Free, Allocated, From, R) ->
   case lists:member({R, From}, Allocated) of
     true ->
         From ! {resource_alloc, yes},
         Allocated1 = lists:delete({R, From}, Allocated),
         case lists:keysearch(From, 2, Allocated1) of
            false -> unlink(From);
            _ -> true
         end,
         server([R|Free], Allocated1);
     ...