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Proof step 15 from proof nodups-butlastn (previous step)

a ∈ butlastn(n, x0), n ≤ # x0, n ≤ # x0 + 1, ∀ n0. ¬ n0 ≤ # x0 ∨ ¬ dups(butlastn(n0, x0)) ⊦ a ∈ x0, dups(x0)

User applied rule weakening

And obtained the following new goals

Proof step 16:
a ∈ butlastn(n, x0), n ≤ # x0 ⊦ a ∈ x0

Rule argument

poslist: left 3, left 4, right 2

Additional information