code: func unpad(p []byte) []byte {
  	c := p[len(p)-1]
  	for i := len(p) - int(c); i < len(p); i++ {
  		if i < 0 || p[i] != c {
  			return nil
  		}
  	}
  	return p[:len(p)-int(c)]
  }
docs: // Removes PKCS #7 standard block padding from p.
  // See http://tools.ietf.org/html/rfc5652#section-6.3.
  // This function is the inverse of pad.
  // If the padding is not well-formed, unpad returns nil.