defp check_input(input, game = %State{}) do
  input = String.trim(input)
  cond do
    input =~ ~r/\A[a-z]\z/ ->
      Map.put(game, :guess, input)
    true ->
      IO.puts "please enter a lower-case letter"
      accept_move(game)
  end
end

def only_single_lowercase(<<c::utf8>>), do: c in ?a..?z