nx.last ∈ nx.butlast ⊦ nx = [], dups(nx)
Used simplifier rules: ⊦ nx.last ∈ nx ↔ nx ≠ []; (s)%bin-01: nx ≠ [] ⊦ n ∈ nx.butlast → n ∈ nx; (forward) from specification list-last[nat-list-set]