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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 _flatten_subclass_tree(cls):
 """Return the set of all child classes of `cls`. Parameters cls : Type Returns ------- frozenset[Type] """

    subclasses = frozenset(cls.__subclasses__())
    children = frozenset(toolz.concat(map(_flatten_subclass_tree, subclasses)))
    return frozenset({cls}) | subclasses | children