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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 _all_reachable_tables(t):
 """ A generator that provides all the names of tables that can be reached via merges starting at the given target table. """

    for k, v in t.items():
        for tname in _all_reachable_tables(v):
            yield tname
        yield k