Map keys and drop unmapped keys with support for nil and false as new keys

mapt.exs

t = %{old1: :new1, old2: :new2, old4: nil, old5: false};
IO.inspect Enum.reduce(
  %{old1: 1, old2: 2, old3: 3, old4: 4, old5: 5},
  %{},
  fn ({old_key, value}, new_map) ->
    case Map.fetch(t, old_key) do
      :error ->
        new_map
      {:ok, new_key} ->
        Map.put(new_map, new_key, value)
    end
  end
)