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Require Import Coq.Program.Tactics.
Require Import Coq.Program.Utils JMeq Lia.
#[local]
Open Scope program_scope.
Inductive vector (A: Type) : nat -> Type :=
| vcons {n:nat} : A -> vector A n -> vector A (S n)
| vnil : vector A 0.
Arguments vcons [A n] _ _.
Arguments vnil {A}.
#[program]
Fixpoint drop
{A: Type}
{n: nat}
(v: vector A n)
(b: nat | b <= n)
{struct v}
: vector A (n - b) :=
match b, v with
| 0, v => v
| S b', vcons _ r => drop r b'
| _, _ => !
end.
Next Obligation. lia. Qed.
Next Obligation. lia. Qed.
Next Obligation. Admitted. (* Can we do better? *)
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