def METHOD_NAME(self, page, ast):
    items = set()
    fields = set()
    for node in moosetree.iterate(ast):
        if node.name == 'TemplateItem':
            items.add(node['key'])
        elif node.name == 'TemplateField':
            fields.add(node['key'])
    unknown_items = items.difference(fields)
    if unknown_items:
        msg = "Unknown template item(s): {}\n{}"
        raise exceptions.MooseDocsException(msg, ', '.join(unknown_items), page.source)