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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 listdir(store, path=None):
 """Obtain a directory listing for the given path. If `store` provides a `listdir` method, this will be called, otherwise will fall back to implementation via the `MutableMapping` interface."""

    path = normalize_storage_path(path)
    if hasattr(store, 'listdir'):
        # pass through
        return store.listdir(path)
    else:
        # slow version, iterate through all keys
        return _listdir_from_keys(store, path)