Def tl(l) == Case of l; nil  nil ; h.t  t

is mentioned by

Thm* it_sum(nil) = 0 Thm* & (l: List. mt(l)  it_sum(l) = head(l)+it_sum(tl(l))) Thm* & (x:, l: List. it_sum(cons(x; l)) = x+it_sum(l))	[it_sum_def]
Def el == n:. l:'a List. if n=0 then hd(l) else el(n-1,tl(l)) fi  Def (recursive)	[hel]
Def tl == l:'a List. tl(l)	[htl]
Def every(p;l) == if null(l) then true else (p(head(l)))every(p;tl(l)) fi  Def (recursive)	[every]
Def map2(f;l1;l2) Def == if null(l1) Def == then nil Def == else if null(l2) Def == else then nil Def == else else cons((f(head(l1),head(l2))); map2(f;tl(l1);tl(l2))) Def == else fi  Def == fi  Def (recursive)	[map2]
Def flatten(l) == if null(l) then nil else head(l) @ flatten(tl(l)) fi  Def (recursive)	[flatten]
Def it_sum(l) == if null(l) then 0 else head(l)+it_sum(tl(l)) fi   (recursive)	[it_sum]

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