b.extensions, b.private = b.extensions[:0], ""
	for _, e := range t.Extensions() {
		b.AddExt(e)
	}
}

// AddExt adds extension e to the tag. e must be a valid extension as returned
// by Tag.Extension. If the extension already exists, it will be discarded,
// except for a -u extension, where non-existing key-type pairs will added.
func (b *Builder) AddExt(e string) {
	if e[0] == 'x' {
		if b.private == "" {
			b.private = e
		}

	b.SetTag(t.locale.Tag())
	if v := tag.Variants(); v != "" {
		for _, v := range strings.Split(v, "-") {
			b.AddVariant(v)
		}
	}
	for _, e := range tag.Extensions() {
		b.AddExt(e)
	}
	return t
}

// FromTag reports closest matching ID for an internal language Tag.
func FromTag(t language.Tag) (id ID, exact bool) {
	// TODO: perhaps give more frequent tags a lower index.

	// TODO: add in alternative variants to -u-va-.
	// TODO: add preferred region to -u-rg-.
	if e := w.Extensions(); len(e) > 0 {
		b := language.Builder{}
		b.SetTag(tt)
		for _, e := range e {
			b.AddExt(e)
		}
		tt = b.Make()
	}
	return makeTag(tt), index, c
}

// ErrMissingLikelyTagsData indicates no information was available
// to compute likely values of missing tags.

	// openssl req -x509 -newkey rsa -keyout key.pem -out cert.pem -days 365 -nodes -subj '/C=US/ST=California/L=San Francisco/O=Internet Widgets, Inc./OU=WWW/CN=Root/emailAddress=******@****.***' -sha256 -addext basicConstraints=CA:TRUE -addext "keyUsage = digitalSignature, keyEncipherment, dataEncipherment, cRLSign, keyCertSign" -utf8