code: func AppendHTMLEscape(dst []byte, s string) []byte {
  	if strings.IndexByte(s, '<') < 0 &&
  		strings.IndexByte(s, '>') < 0 &&
  		strings.IndexByte(s, '"') < 0 &&
  		strings.IndexByte(s, '\'') < 0 {
  
  		// fast path - nothing to escape
  		return append(dst, s...)
  	}
  
  	// slow path
  	var prev int
  	var sub string
  	for i, n := 0, len(s); i < n; i++ {
  		sub = ""
  		switch s[i] {
  		case '<':
  			sub = "&lt;"
  		case '>':
  			sub = "&gt;"
  		case '"':
  			sub = "&quot;"
  		case '\'':
  			sub = "&#39;"
  		}
  		if len(sub) > 0 {
  			dst = append(dst, s[prev:i]...)
  			dst = append(dst, sub...)
  			prev = i + 1
  		}
  	}
  	return append(dst, s[prev:]...)
  }
docs: // AppendHTMLEscape appends html-escaped s to dst and returns the extended dst.