// start is the opening tag of the element.
func (d *Decoder) unmarshalInterface(val Unmarshaler, start *StartElement) error {
	// Record that decoder must stop at end tag corresponding to start.
	d.pushEOF()

	d.unmarshalDepth++
	err := val.UnmarshalXML(d, *start)
	d.unmarshalDepth--
	if err != nil {
		d.popEOF()
		return err
	}

	if !d.popEOF() {

		d.free = s
	}
	return s
}

// Record that after the current element is finished
// (that element is already pushed on the stack)
// Token should return EOF until popEOF is called.
func (d *Decoder) pushEOF() {
	// Walk down stack to find Start.
	// It might not be the top, because there might be stkNs
	// entries above it.
	start := d.stk
	for start.kind != stkStart {
		start = start.next
	}