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Proof step 2 from proof first-append (previous step)

m ≤ # x ⊦ firstn(m, x + y) = firstn(m, x)

User applied rule structural induction

And obtained the following new goals

Proof step 3:
∀ x, y. m0 ≤ # x → firstn(m0, x + y) = firstn(m0, x), m0 +1 ≤ # x ⊦ firstn(m0 +1, x + y) = firstn(m0 +1, x)

Rule argument

indvar: m

Additional information

Used simplifier rules: firstN-zero: ⊦ firstn(0, x) = [];%nat freely generated by 0, +1;;