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Proof step 1 from proof lastn-firstn-in-01 (conclusion)

⊦ lastn(n, x) = y ∧ ¬ a ∈ y ∧ a ∈ x ∧ n ≤ # x ∧ m = # x - n → a ∈ firstn(m, x)

System applied rule normalize

And obtained the following new goals

Proof step 2:
lastn(n, x) = y, m = # x - n, a ∈ x, n ≤ # x ⊦ a ∈ firstn(m, x), a ∈ y

Rule argument

Additional information