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Proof step 3 from proof firstN-lastN-in-01 (previous step)

⊦ n ≤ # x ∧ a ∈ x ∧ ¬ a ∈ lastn(n, x) → (¬ a ∈ firstn(# x - n, x) ↔ false)

Lemma firstN-lastN-in