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Proof step 2 from proof diag-theorem-butlast (previous step)

∀ n, a. nextl(n, a) ≠ [] ∧ nextl(n, a).first = a ⊦ ∃ σ, ii. ii(0) = 0 ∧ σ[0] = a ∧ σ ≠ [] ∧ (∀ n. ii(n + 1) = ii(n) + # nextl(n, σ[ii(n)]) - 1 ∧ ⌜ii(n + 1)⌝ < # σ ∧ (σ from ii(n)) to # nextl(n, σ[ii(n)]) = ⌜nextl(n, σ[ii(n)])⌝ ) ∧ (finp(σ) ⊃ ∃ n. # σ = ⌜ii(n)⌝ + ⌜1⌝ ∧ (∀ m. n ≤ m → nextl(m, σ[ii(m)]) = σ[ii(m)] ' ∧ ii(n) = ii(m));∀ n. ∃ m. ii(m) ≤ n ∧ n < ii(m + 1))

User applied rule insert lemma

And obtained the following new goals

Proof step 3:
∀ n, a. nextl(n, a) ≠ [] ⊦ ∃ lf. lf(0) = a ' ∧ (∀ n. lf(n + 1) = nextl(n, lf(n).last) ∧ lf(n) ≠ [])
Proof step 4:
∀ n, a. nextl(n, a) ≠ [] ∧ nextl(n, a).first = a ⊦ ∀ n, a. nextl(n, a) ≠ [], ∃ σ, ii. ii(0) = 0 ∧ σ[0] = a ∧ σ ≠ [] ∧ (∀ n. ii(n + 1) = ii(n) + # nextl(n, σ[ii(n)]) - 1 ∧ ⌜ii(n + 1)⌝ < # σ ∧ (σ from ii(n)) to # nextl(n, σ[ii(n)]) = ⌜nextl(n, σ[ii(n)])⌝ ) ∧ (finp(σ) ⊃ ∃ n. # σ = ⌜ii(n)⌝ + ⌜1⌝ ∧ (∀ m. n ≤ m → nextl(m, σ[ii(m)]) = σ[ii(m)] ' ∧ ii(n) = ii(m));∀ n. ∃ m. ii(m) ≤ n ∧ n < ii(m + 1))
Proof step 6:
∃ lf. lf(0) = a ' ∧ (∀ n. lf(n + 1) = nextl(n, lf(n).last) ∧ lf(n) ≠ []), ∀ n, a. nextl(n, a) ≠ [] ∧ nextl(n, a).first = a ⊦ ∃ σ, ii. ii(0) = 0 ∧ σ[0] = a ∧ σ ≠ [] ∧ (∀ n. ii(n + 1) = ii(n) + # nextl(n, σ[ii(n)]) - 1 ∧ ⌜ii(n + 1)⌝ < # σ ∧ (σ from ii(n)) to # nextl(n, σ[ii(n)]) = ⌜nextl(n, σ[ii(n)])⌝ ) ∧ (finp(σ) ⊃ ∃ n. # σ = ⌜ii(n)⌝ + ⌜1⌝ ∧ (∀ m. n ≤ m → nextl(m, σ[ii(m)]) = σ[ii(m)] ' ∧ ii(n) = ii(m));∀ n. ∃ m. ii(m) ≤ n ∧ n < ii(m + 1))

Rule argument

spec: infinite-append inst: name: nextl-lemma-butlast-nonempty seq: ∀ n, a. nextl(n, a) ≠ [] ⊦ ∃ lf. lf(0) = a ' ∧ (∀ n. lf(n + 1) = nextl(n, lf(n).last) ∧ lf(n) ≠ []) subst: [nextl, a] → [nextl, a] flags: current, []

Additional information

Used terms: nextl, a