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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 get_waveset(model):
 """Get optimal wavelengths for sampling a given model. Parameters model : `~astropy.modeling.Model` Model. Returns ------- waveset : array-like or `None` Optimal wavelengths. `None` if undefined. Raises ------ synphot.exceptions.SynphotError Invalid model. """

    if not isinstance(model, Model):
        raise SynphotError('{0} is not a model.'.format(model))

    if isinstance(model, _CompoundModel):
        waveset = model._tree.evaluate(WAVESET_OPERATORS, getter=None)
    else:
        waveset = _get_sampleset(model)

    return waveset