def get_config_files():
        """
        Return the application configuration files.

        Return a list of configuration files describing the apps supported by
        Mackup. The files return are absolute full path to those files.
        e.g. /usr/lib/mackup/applications/bash.cfg

        Only one config file per application should be returned, custom config
        having a priority over stock config.

        Returns:
            set of strings.
        """
        # Configure the config parser
        apps_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)),
                                APPS_DIR)
        custom_apps_dir = os.path.join(os.environ['HOME'], CUSTOM_APPS_DIR)

        # List of stock application config files
        config_files = set()

        # Temp list of user added app config file names
        custom_files = set()

        # Get the list of custom application config files first
        if os.path.isdir(custom_apps_dir):
            for filename in os.listdir(custom_apps_dir):
                if filename.endswith('.cfg'):
                    config_files.add(os.path.join(custom_apps_dir,
                                                  filename))
                    # Also add it to the set of custom apps, so that we don't
                    # add the stock config for the same app too
                    custom_files.add(filename)

        # Add the default provided app config files, but only if those are not
        # customized, as we don't want to overwrite custom app config.
        for filename in os.listdir(apps_dir):
            if filename.endswith('.cfg') and filename not in custom_files:
                config_files.add(os.path.join(apps_dir, filename))

        return config_files
Return the application configuration files.

        Return a list of configuration files describing the apps supported by
        Mackup. The files return are absolute full path to those files.
        e.g. /usr/lib/mackup/applications/bash.cfg

        Only one config file per application should be returned, custom config
        having a priority over stock config.

        Returns:
            set of strings.