to acer:

Yes, it is great.

Thanks so much.

Thanks, acer,

I find another way to solve this problem. here is the way:

> F:=proc(L)

>local LL;

>   LL:=subsop(1=NULL,LL);

>L:=LL;
> end proc:

> L:=[x,y,z,d];
                               L := [x, y, z, d]

>L:= F(L);
                                   [y, z, d]
 >L:= F(L);
                                    [z, d]
 >L:= F(L);
                                      [d]
 >F(L);
                                      []
 

By the way, shy it is evil? if I want to such thing, whether there are other methods that are not evil:)