code: public static Operator determineOperator(final String operatorId,
                                               final boolean isNegated) {
          Operator op = CACHE.get( getKey( operatorId,
                                           isNegated ) );
          return op;
      }
docs: Returns the operator instance for the given parameters
  
  @param operatorId the identification symbol of the operator
  @param isNegated true if it is negated
  
  @return the operator in case it exists