A generator that provides all the names of tables that can be
    reached via merges starting at the given target table.

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