code: public GroupBuilder addVariable(String name, String property, String className, DJCalculation operation) {		
  		group.getVariables().add(new DJGroupVariableDef(name, new ColumnProperty(property,className), operation));
  		return this;
  	}
docs: Registers a variable for later reference inside custom expressions.
  The new variable will perform the calculation using the field information passed in the ColumnProperty
  parameter. Such ColumnProperty will be properly registered in the report design as a field ($F{...})
  
  
  @param name
  @param property
  @param className
  @param operation
  @return