@classmethod
def extra_key_files(cls):
    """
        Return a list of extra key file paths (or path glob patterns) beyond
        standard, well known key files for this Package. List items are strings
        that are either paths or glob patterns and are relative to the package
        root.

        Knowing if a file is a "key-file" file is important for classification
        and summarization. For instance, a JAR can have key files that are not
        top level under the META-INF directory. Or a .gem archive contains a
        metadata.gz file.

        Sub-classes can implement as needed.
        """
    return []
