∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} [inst : EquivBEq α] [inst_1 : LawfulHashable α]
  [inst_2 : Inhabited α] {l : List ((a : α) × β a)} {k k' : α},
  (k == k') = true →
    List.Pairwise (fun a b => (a.fst == b.fst) = false) l →
      k ∈ List.map Sigma.fst l → (Std.ExtDHashMap.ofList l).getKey! k' = k