code: def register_pivot_wavelength(self, telescope, band, wlen):
          
          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
docs: Register precomputed pivot wavelengths.