code: func (t *SimpleStyledText) RegisterStyle(r rune, style tcell.Style) {
  	if r == 'N' {
  		t.Text.SetStyle(style)
  	}
  	if unicode.IsLetter(r) {
  		t.styles[r] = style
  	}
  }
docs: // Registers a style for the given rune.  This style will be used for
  // text marked with %<r>. See SetMarkup() for more detail.  Note that
  // this must be done before using any of the styles with SetMarkup().
  // Only letters may be used when registering styles, and be advised that
  // the system may have predefined uses for upper case letters.