def METHOD_NAME(self, xsd_element_node, xml_parent_node):
    # The attribute @default of the xsd:element node
    v = xsd_element_node.get("default")
    if v is not None:
        return v
    # The acrn:defaults and acrn:unique-among annotations which define a set of default values that shall be unique
    # among a collection of nodes
    annot_node = self.get_node(xsd_element_node, "xs:annotation")
    if annot_node is not None:
        defaults = annot_node.get("{https://projectacrn.org}defaults")
        unique_among = annot_node.get("{https://projectacrn.org}unique-among")
        if defaults is not None and unique_among is not None:
            try:
                default_values = set(eval(defaults))
                existing_values = set(elementpath.select(self.xml_etree, unique_among, variables={"parent": xml_parent_node}))
                available_defaults = default_values - existing_values
                return sorted(list(available_defaults))[0]
            except:
                pass
    return None