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Returns None on last element of hlist.
A correct solution
Require Import List.
Set Implicit Arguments.
Inductive hlist : list Set -> Type :=
| hnil : hlist nil
| hcons {X : Set} {Xs : list Set} (x : X) :
hlist Xs -> hlist (X :: Xs).
Notation "[;;]" := (hnil).
Notation "h ;; t" := (hcons h t)
(at level 30, right associativity).
Fixpoint hget
{Xs : list Set}
(hls : hlist Xs)
(n : nat)
: option (nth n Xs Empty_set) :=
match n with
| O =>
match hls with
| hnil => None
| hcons x _ => Some x
end
| S n =>
match hls with
| hnil => None
| @hcons _ Xs _ hls => @hget Xs hls n
end
end.
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