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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 register_pivot_wavelength(self, telescope, band, wlen):
 """Register precomputed pivot wavelengths."""

        if (telescope, band) in self._pivot_wavelengths:
            raise AlreadyDefinedError('pivot wavelength for %s/%s already '
                                      'defined', telescope, band)
        self._note(telescope, band)
        self._pivot_wavelengths[telescope,band] = wlen
        return self