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Proof step 7 from proof firstN-lastN-in (previous step)

a ∈ firstn(# x - n, x) + lastn(n, x), n ≤ # x ⊦ a ∈ firstn(# x - n, x), a ∈ lastn(n, x)

User applied rule insert rewrite lemma

And obtained the following new goals

Proof step 8:
⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y

Rule argument

spec: list name: in-append seq: ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y subst: [y, x, a] → [lastn(n, x), firstn(# x - n, x), a] flags: <all paths>

Additional information

Used terms: lastn(n, x), firstn(# x - n, x), a