// rest.
func Decode(data []byte) (p *Block, rest []byte) {
	// pemStart begins with a newline. However, at the very beginning of
	// the byte array, we'll accept the start string without it.
	rest = data
	for {
		if bytes.HasPrefix(rest, pemStart[1:]) {
			rest = rest[len(pemStart)-1:]
		} else if _, after, ok := bytes.Cut(rest, pemStart); ok {
			rest = after
		} else {
			return nil, data
		}