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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 _resolve_subkeys(key, separator='.'):
 """Given a key which may actually be a nested key, return the top level key and any nested subkeys as separate values. Args: key (str):
 A string that may or may not contain the separator. separator (str):
 The namespace separator. Defaults to `.`. Returns: Tuple[str, str]: The key and subkey(s). """

    subkey = None
    if separator in key:
        index = key.index(separator)
        subkey = key[index + 1:]
        key = key[:index]
    return key, subkey