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Proof step 1 from proof blast-first (conclusion)

⊦ a ∈ x.butlast ∧ x ≠ [] → x.butlast.first = x.first

Heuristic simplifier applied rule simplifier

And obtained the following new goals

Proof step 2:
a ∈ x.butlast, a ∈ x ⊦ x.butlast.first = x.first, x = []

Rule argument

Additional information

Used simplifier rules: bin-01: x ≠ [] ⊦ a ∈ x.butlast → a ∈ x;