code: public double get(Set presentFields) {
      int index = 0;
      for (int f = 0; f < indexFields.length; ++f) {
        String field = indexFields[f];
        index *= 2;
        if (presentFields.contains(field)) {
          ++index;
        }
      }
      return matrix[index];
    }
docs: Map<String, boolean>