code: public Evaluator getAttributeEvaluator(String name, String mode, String value)
              throws Exception {
          return new AttributeEvaluator(name, mode, value);
      }
docs: returns an Evaluator unique to the type of attribute being evaluated. subclasses can override
  this method to return the Evaluator that's appropriate to their implementation.
  
  @param name the attribute's name.
  @param mode the attribute's mode. (i.e. 'equals')
  @param value the attribute's value.
  @return an Evaluator for evaluating attributes