code: def relevantindices(self) -> List[int]:
          
          return [idx for idx in numpy.unique(self.refindices.values)
                  if idx in self.RELEVANT_VALUES]
docs: A |list| of all currently relevant indices, calculated as an
          intercection of the (constant) class attribute `RELEVANT_VALUES`
          and the (variable) property |IndexMask.refindices|.