Def null(as) == Case of as; nil  true ; a.as'  false

is mentioned by

Thm* l:'a List. null(l)  mt(l)	[assert_of_null_is_mt]
Def hd == l:'a List. if null(l) then arb('a) else head(l) fi	[hhd]
Def null == l:'a List. null(l)	[hnull]
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]

In prior sections: list 1

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