// so far. Reading more content changes the MD5
// checksum. Therefore, calling ETag multiple
// times may return different results.
func (r *Reader) ETag() ETag {
	sum := r.md5.Sum(nil)
	return ETag(sum)
}

// VerifyError is an error signaling that a
// computed ETag does not match an expected
// ETag.
type VerifyError struct {
	Expected ETag
	Computed ETag
}