// It is mainly used to provide a high-level io.Reader
// access to the ETag computed by a low-level io.Reader:
//
//	content := etag.NewReader(r.Body, nil)
//
//	compressedContent := Compress(content)
//	encryptedContent := Encrypt(compressedContent)
//
//	// Now, we need an io.Reader that can access
//	// the ETag computed over the content.
//	reader := etag.Wrap(encryptedContent, content)