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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 _derive_typedef(typ):
    '''Return single, existing super type typedef or None.
    '''
    v = [v for v in _values(_typedefs) if _issubclass(typ, v.type)]
    if len(v) == 1:
        return v[0]
    return None