I'm with Brian on this, although it the "tailrec" specification needs to 
be optional.  I use tail recursion by accident a lot when I'm coding 
("Oh, hey, I guess this is tail recursive.  Nifty!"), and I would be 
really annoyed to have to think about whether my recursion is tail 
recursive or not every time I use it.

~~ Robert.
>
> A better proposal would be one where the call itself is marked as a 
> tailcall, and bombs out if that call isn't.  Maybe something like:
>
>    let rec loop i s =
>       if i > 0 then
>          tailcall loop (i-1) (s+i) (* force this to be a tailcall *)
>       else s
>    in
>    let t = 2 * (loop n 0) in ... (* not a tail call, and that's OK *)
>
> At which point I'm neutral on the idea.  The rules for what is or 
> isn't a tail call are not complicated.  On the other hand, it's a 
> little bit of extra correctness for not much cost.
>
> Brian