code: private Map<String, Map<String, Row>> getPPMatrixMap(){
  
  		if(this.ppMatrixMap == null){
  			this.ppMatrixMap = getValue(GeneralRegressionModelEvaluator.ppMatrixCache);
  		}
  
  		return this.ppMatrixMap;
  	}
docs: <p>
  A PPMatrix element may encode zero or more matrices.
  Regression models return a singleton map, whereas classification models
  may return a singleton map or a multi-valued map, which overrides the default
  matrix for one or more target categories.
  </p>
  
  <p>
  The default matrix is mapped to the <code>null</code> key.
  </p>
  
  @return A map of predictor-to-parameter correlation matrices.