∀ {α : Type u} {β : α → Type v} [inst : BEq α] [inst_1 : LawfulBEq α] {k : α} [inst_2 : Inhabited (β k)] {f : β k → β k}
  (l : List ((a : α) × β a)),
  Std.Internal.List.DistinctKeys l →
    Std.Internal.List.getValueCast! k (Std.Internal.List.modifyKey k f l) =
      (Option.map f (Std.Internal.List.getValueCast? k l)).get!