match l with
[ nil ⇒ \ 5a title="natural number" href="cic:/fakeuri.def(1)"\ 60\ 5/a\ 6
| cons a tl ⇒ \ 5a href="cic:/matita/arithmetics/nat/nat.con(0,2,0)"\ 6S\ 5/a\ 6 (length A tl)].
match l with
[ nil ⇒ \ 5a title="natural number" href="cic:/fakeuri.def(1)"\ 60\ 5/a\ 6
| cons a tl ⇒ \ 5a href="cic:/matita/arithmetics/nat/nat.con(0,2,0)"\ 6S\ 5/a\ 6 (length A tl)].