docs: def _to_wcxf(self, C_out, scale_out):
          """Return the Wilson coefficients `C_out` as a wcxf.WC instance.
  
          Note that the Wilson coefficients are rotated into the Warsaw basis
          as defined in WCxf, i.e. to the basis where the down-type and charged
          lepton mass matrices are diagonal."""
          C = self._rotate_defaultbasis(C_out)
          d = wilson.util.smeftutil.arrays2wcxf_nonred(C)
          basis = wcxf.Basis['SMEFT', 'Warsaw']
          all_wcs = set(basis.all_wcs)  # to speed up lookup
          d = {k: v for k, v in d.items() if k in all_wcs and v != 0}
          d = wcxf.WC.dict2values(d)
          wc = wcxf.WC('SMEFT', 'Warsaw', scale_out, d)
          return wc