def check_element(self, elem, check_children=False, next_to_elem=None):
        """
        Given an element, check its attributes for references to the three proton attributes ('eid', 'aid' and 'rid').
        """
        self.__add_element('eid', elem.attribs, self.__element_ids, elem, next_to_elem)
        self.__add_element('aid', elem.attribs, self.__attrib_ids, elem, next_to_elem)
        self.__add_element('rid', elem.attribs, self.__repeat_ids, elem, next_to_elem)
        if check_children and elem.children:
            for child in elem.children:
                self.check_element(child, True, next_to_elem)