list a ::= Nil | Cons a (list a); ;;
filter p l
= case l of Nil -> Nil;
Cons x xs -> case p x of
True -> Cons x (filter p xs);
False -> filter p xs
end
end;
empty l = case l of Nil -> True; Cons x xs -> False end;
llist = Cons (Cons 1 Nil) Nil;
res = filter empty llist;
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