func isValid(code string) bool {
	st := make([]string, 0)
	for i := 0; i < len(code); i++ {
		if 0 < i && len(st) == 0 { // Unmatched content
			return false
		}
		if strings.HasPrefix(code[i:], "<![CDATA[") { // Parse cdata
			j := strings.Index(code[i+9:], "]]>")
			if j < 0 {
				return false
			}
			i += j + 11
		} else if strings.HasPrefix(code[i:], "</") { // Parse end
			i += 2
			j := strings.Index(code[i:], ">")
			if j < 0 {
				return false
			}
			name := code[i : i+j]
			if l := len(st); l == 0 || st[l-1] != name {
				return false
			} else {
				st = st[:l-1]
			}
			i += j
		} else if code[i] == '<' { // Parse begin
			i += 1
			j := strings.Index(code[i:], ">")
			if j < 1 || 9 < j {
				return false
			}
			name := code[i : i+j]
			for _, r := range name {
				if r < 'A' || 'Z' < r {
					return false
				}
			}
			st = append(st, name)
			i += j
		}
	}
	return len(st) == 0
}