code: public List<org.apache.lucene.search.SortField> sortFields(Schema schema) {
          return sort.stream().map(s -> s.sortField(schema)).collect(Collectors.toList());
      }
docs: Returns the Lucene {@link org.apache.lucene.search.SortField}s represented by this using the specified schema.
  
  @param schema the indexing schema to be used
  @return the Lucene sort fields represented by this using {@code schema}