∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.HashMap α β} [EquivBEq α] [LawfulHashable α]
  {k k' : α} {fallback : β} {f : Option β → Option β},
  (m.alter k f).getD k' fallback = if (k == k') = true then (f m[k]?).getD fallback else m.getD k' fallback