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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 _call(self, x):
 """Return wavelet transform of ``x``."""

        if self.impl == 'pywt':
            coeffs = pywt.wavedecn(
                x, wavelet=self.pywt_wavelet, level=self.nlevels,
                mode=self.pywt_pad_mode, axes=self.axes)
            return pywt.ravel_coeffs(coeffs, axes=self.axes)[0]
        else:
            raise RuntimeError("bad `impl` '{}'".format(self.impl))