(*
 * These definitions of list-handling functions avoid all run-time error
 * messages.  Note that for specl, the former type name ?T has been changed to
 * just T.
 *)

obj T = integer;

function head(l:T*) =
    if #l = 0 then nil
    else l[1];

function tail(l:T*) =
    if (#l = 0) or (#l = 1) then nil
    else l[2..#l];

function last(l:T*) =
    if (#l = 0) then nil
    else l[#l];

function lastn(l:T*, n:integer) =
    if (n <= 0) or (n > #l) then nil
    else l[#l-n+1..#l];

> head([]);
> tail([]);
> last([]);
> lastn([], 0);

> head([1]);
> tail([1]);
> last([1]);
> lastn([1],1);

> head([1,2]);
> tail([1,2]);
> last([1,2]);
> lastn([1,2], 1);
> lastn([1,2], 2);
> lastn([1,2], 3);

> "1, [2,3], 3, nil, [3], [2,3], [1,2,3], nil:";
> head([1,2,3]);
> tail([1,2,3]);
> last([1,2,3]);
> lastn([1,2,3], 0);
> lastn([1,2,3], 1);
> lastn([1,2,3], 2);
> lastn([1,2,3], 3);
> lastn([1,2,3], 4);