docs: def get_permissible_wcs(classname, f):
      r"""For some classes (in particular $\Delta F=1$), only a subset of Wilson
      coefficients exist in WET-3 and WET-4. Therefore, depending on the number
      of flavours `f`, the dimensionality of the ADM has to be reduced."""
      # these are the problematic sectors
      classes = ['cu', 'db', 'sb', 'sd', 'mue', 'mutau', 'taue', 'dF0']
      if classname not in classes or f == 5:
          # for 5-flavour WET, nothing to do.
          # Neither for other classes (I, II, ...) because they exist either all
          # or not at all in WET-3 and WET-4 (they have specific flavours).
          return 'all'
      if f not in [3, 4]:
          raise ValueError("f must be 3, 4, or 5.")
      if classname == 'dF0':
          sector = 'dF=0'
      else:
          sector = classname
      perm_keys = wcxf.Basis['WET-{}'.format(f), 'JMS'].sectors[sector].keys()
      all_keys = coeffs[sector]
      return [i for i, c in enumerate(all_keys) if c in perm_keys]