Ch2, Ex10

declare
proc {SMerge Xs Ys ?R}
   local T in
      T = Xs#Ys
      case T
      of nil#Ys then R = Ys
      else
         case T of Xs#nil then R = Xs
         else
            case T of
               (X|Xr)#(Y|Yr) then
               local K in
                  K = X=<Y
                  if K then S in
                     R = X|S
                     {SMerge Xr Ys S}
                  else S in
                     R = Y|S
                     {SMerge Xr Ys S}
                  end
               end
            end
         end
      end      
   end
end

Notice the usage of dataflow variable S to keep recursive call to be last so as to make it tail-recursive, this trick is used when a function call is made inside a data-structure(for reference read the section "Function calls in data structures" in the book).