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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_default_config_file(rootdir=None):
 """Search for configuration file."""

    if rootdir is None:
        return DEFAULT_CONFIG_FILE

    for path in CONFIG_FILES:
        path = os.path.join(rootdir, path)
        if os.path.isfile(path) and os.access(path, os.R_OK):
            return path