private void TryLetter(string letter)
        {
            if (Modal.IsVisible)
            {
                return;
            }

            Letters.MakeInvisible(letter);
            if (Secret.MakeVisible(letter))
            {
                if (Secret.IsAllVisible)
                {
                    SetModalText("You win");
                }
                return;
            }

            if (Gallows.MakeVisible())
            {
                SetModalText(String.Join("", Secret.Word.Select(pair => pair.Text)));
            }
        }