∀ {α : Type u} {β : α → Type v} [inst : BEq α] [EquivBEq α] {l₁ : List ((a : α) × β a)} {l₂ : List α} {k : α},
  Std.Internal.List.DistinctKeys l₁ →
    ∀ {h₁ : Std.Internal.List.containsKey k (List.filter (fun p => !l₂.contains p.fst) l₁) = true}
      {h₂ : Std.Internal.List.containsKey k l₁ = true},
      Std.Internal.List.getKey k (List.filter (fun p => !l₂.contains p.fst) l₁) h₁ = Std.Internal.List.getKey k l₁ h₂