internal/unicode/cesu8/cesu8.go
func FullRune(p []byte) bool {
	high, n, short := decodeRune(p)
	if short {
		return false
	}
	if !utf16.IsSurrogate(high) {
		return true
	}
	_, _, short = decodeRune(p[n:])
	return !short
}
func FullRune(p []byte) bool {
	high, n, short := decodeRune(p)
	if short {
		return false
	}
	if !utf16.IsSurrogate(high) {
		return true
	}
	_, _, short = decodeRune(p[n:])
	return !short
}
// FullRune reports whether the bytes in p begin with a full CESU-8 encoding of a rune.