code: public Evaluator getEvaluator( ValueType type,
                                     Operator operator ) {
          return this.getEvaluatorDefinition( operator ).getEvaluator( type,
                                                                       operator );
      }
docs: Returns the evaluator instance for the given type and the
  defined parameterText
  
  @param type the type of the attributes this evaluator will
  operate on. This is important because the evaluator
  may do optimizations and type coercion based on the
  types it is evaluating. It is also possible that
  this evaluator does not support a given type.
  
  @param operator the operator that evaluator implements
  
  @return an Evaluator instance capable of evaluating expressions
  between values of the given type, or null in case the type
  is not supported.