[ nil ⇒ 0
| cons a tl ⇒ S (length A tl)].
-notation "|M|" non associative with precedence 65 for @{'norm $M}.
-interpretation "norm" 'norm l = (length ? l).
+interpretation "list length" 'card l = (length ? l).
lemma length_tail: ∀A,l. length ? (tail A l) = pred (length ? l).
#A #l elim l //