// copied from a Tag in SetTag.
func (b *Builder) ClearVariants() {
	b.variants = b.variants[:0]
}

// ClearExtensions removes any extensions previously added, including those
// copied from a Tag in SetTag.
func (b *Builder) ClearExtensions() {
	b.private = ""
	b.extensions = b.extensions[:0]
}

func tokenLen(token ...string) (n int) {
	for _, t := range token {

				err = errInvalidArgument
				break
			}
			b.SetExt(v.s)
		case []Variant:
			b.ClearVariants()
			for _, v := range v {
				b.AddVariant(v.variant)
			}
		case []Extension:
			b.ClearExtensions()
			for _, e := range v {
				b.SetExt(e.s)
			}
		// TODO: support parsing of raw strings based on morphology or just extensions?
		case error:
			if v != nil {
				err = v
			}
		}
	}