I have this XML file:

<collection>
    <journal name = "La Repubblica" issn = "1">
        <volume number = "1" date = "12/04/2022">
            <article doi = "AB23">
                <author orcid = "XY21">
                    <firstname>Marco</firstname>
                    <surname>Rossi</surname>
                    <affiliation>Affi</affiliation>
                </author>
                <author orcid = "XYZ">
                    <firstname>Simone</firstname>
                    <surname>Mambo</surname>
                </author>
                <title>Stop alle mascherine</title>
                <abstract>Da maggio si può dire basta alle mascherine</abstract>
                <keyword>COVID</keyword>
            </article>
            <article doi = "AB60">
                <author orcid = "XY21">
                    <firstname>Marco</firstname>
                    <surname>Rossi</surname>
                    <affiliation>Affi</affiliation>
                </author>
                <title>Un altro titolo</title>
                <abstract>Un altro abstract</abstract>
                <keyword>Titolo</keyword>
            </article>
        </volume>
    </journal>
</collection>

and this xQuery script:

<publications>
{
    for $au in distinct-values(doc("collection.xml")//author)
    return
    <author orcid = "{ $au/@orcid }">
       <firstname>{ $au/firstname/text() }</firstname>
    </author>
}
</publications>

Unfortunately this is giving me:

XPTY0019 The required item type of the first operand of “https://stackoverflow.com/” is node(), but the supplied
expression {$ar} has item type xs:anyAtomicType