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Proof step 3 from proof firstn-twice (previous step)

n ≤ # x ⊦ firstn(n, firstn(n, x)) = firstn(n, x)

User applied rule structural induction

And obtained the following new goals

Proof step 4:
n ≤ #(a + x0), ∀ n. n ≤ # x0 → firstn(n, firstn(n, x0)) = firstn(n, x0) ⊦ firstn(n, firstn(n, a + x0)) = firstn(n, a + x0)

Rule argument

indvar: x

Additional information

Used simplifier rules: firstN-zero: ⊦ firstn(0, x) = [];%list freely generated by [], +;;% ⊦ # [] = 0; (s)% ⊦ m ≤ 0 ↔ m = 0; (s)