code: protected Retriever createRetriever()
      {
          return new Retriever()
          {
              @Override
              public String retrieve(String name)
              {
                  return properties.getProperty(name);
              }
          };
      }
docs: Build a new {@link Retriever} instance that will be used by the
  {@link #getEvaluator() evaluator} to request the values of nested property
  references.
  
  @return a new {@link Retriever}