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Proof step 1 from proof firstN-lastN-in-01 (conclusion)

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

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 2:
a ∈ x, n < # x ⊦ x = [], a ∈ firstn(# x - n, x), a ∈ lastn(n, x)

Rule argument

Additional information

Used simplifier rules: l: ⊦ # x = 0 ↔ x = []; from specification list-data%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1