code: func wrap(sl []byte) []byte {
  	length := 0
  	for i := 0; i < len(sl); i++ {
  		if length > 76 && sl[i] == ' ' {
  			sl = append(sl, 0, 0)
  			copy(sl[i+2:], sl[i:])
  			sl[i] = '\r'
  			sl[i+1] = '\n'
  			sl[i+2] = '\t'
  			i += 2
  			length = 0
  		}
  		if sl[i] == '\n' {
  			length = 0
  		}
  		length++
  	}
  	return sl
  }
docs: // Wrap a byte slice paragraph for use in SMTP header