XPath expression which expresses sequence-extended =

Hi Folks,

Consider this XML:

<sequences>
    <sequence>
        <item>A</item>
    </sequence>
    <sequence>
        <item>A</item>
        <item>B</item>
    </sequence>
</sequences>

Suppose that $sequence-extended has this value:

    <sequence>
        <item>A</item>
        <item>B</item>
    </sequence>

And $sequence has this value:

    <sequence>
        <item>A</item>
    </sequence>

And $item has this value:

    <item>B</item>

I want an XPath expression which expresses this:

    The $sequence-extended items equals
    the $sequence items plus $item.

This XPath expression seems to work:

$sequence-extended/item = ($sequence/item, $item)

But, the XPath expression fails when $sequence is empty:

    <sequence/>

And $sequence-extended has a single item:

    <sequence>
        <item>A</item>
    </sequence>

And $item is:

    <item>A</item>

What XPath 2.0 expression works in all cases?

Note: I seek simplicity and plainness over efficiency and cleverness.

/Roger