for n < len(s) {
		if c := s[n]; c < utf8.RuneSelf {
			if isIdentASCII(c) && (n > 0 || c < '0' || c > '9') {
				n++
				continue
			}
			break
		}
		r, nr := utf8.DecodeRuneInString(s[n:])
		if unicode.IsLetter(r) {
			n += nr
			continue
		}
		break
	}
	return s[:n], n > 0
}

// isIdentASCII reports whether c is an ASCII identifier byte.