const unicode.MaxRune

4 uses

	unicode (current package)
		letter.go#L10: 	MaxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
		letter.go#L83: 	UpperLower = MaxRune + 1 // (Cannot be a valid delta.)
		letter.go#L233: 	if delta > MaxRune {
		letter.go#L355: 	if r < 0 || r > MaxRune {