code: func (s *SelectionPredicate) MatcherIndex() []MatchValue {
  	var result []MatchValue
  	for _, field := range s.IndexFields {
  		if value, ok := s.Field.RequiresExactMatch(field); ok {
  			result = append(result, MatchValue{IndexName: field, Value: value})
  		}
  	}
  	return result
  }
docs: // For any index defined by IndexFields, if a matcher can match only (a subset)
  // of objects that return <value> for a given index, a pair (<index name>, <value>)
  // wil be returned.
  // TODO: Consider supporting also labels.