code: public void addWrittenFields(FieldSet writtenFields) {
  		if(this.writtenFields == null) {
  			this.writtenFields = new FieldSet(writtenFields);
  		} else {
  			this.writtenFields.addAll(writtenFields);
  		}
  	}
docs: Adds, to the existing information, field(s) that are written in
  the destination record(s).
  
  @param writtenFields the position(s) in the destination record(s)