There is a way to remove an attribute completely:

<xpath expr="//field[@name='order_line']/tree" position="attributes">

    <attribute name="editable"/>

</xpath>

And there is a way to completely replace an attribute with smth else:

<xpath expr="//label[@for='type']" position="attributes">

    <attribute name="groups">sale.group_delivery_invoice_address</attribute>

    <attribute name="invisible">False</attribute>

</xpath>

But is there a short way to append a value to an attribute?

So that after appending my_second_class to attribute class in <div class="my_first_class"> we receive <div class="my_first_class my_second_class">.