def available_configuration_files(self):
        """A list of strings with the absolute pathnames of the available configuration files."""
        known_files = [GLOBAL_CONFIG, LOCAL_CONFIG, self.environment.get('PIP_ACCEL_CONFIG')]
        absolute_paths = [parse_path(pathname) for pathname in known_files if pathname]
        return [pathname for pathname in absolute_paths if os.path.isfile(pathname)]
A list of strings with the absolute pathnames of the available configuration files.