let compare kf1 kf2 =
         if kf1 == kf2 then 0
         else
           let res =
             String.compare
               (Kernel_function.get_name kf1)
               (Kernel_function.get_name kf2)
           in
           if res <> 0 then res
           else
             String.compare
               (Kernel_function.get_vi kf1).vname
               (Kernel_function.get_vi kf2).vname