anchor.diff:  
   		public
   			constructor create();
  +			destructor  destroy();
   
   			function start(): Iterator;
   			function finish(): Iterator;
  
positive.code: 		prevIt, currentIt: Iterator;
  	begin
  		currentIt := Iterator.create(element);
  		if head = tail then
  		begin
  			head := currentIt;
  			head.setNext(tail);
  
negative.code: 	begin
  		if p = tail then
  			exit();
  		if p = start() then
  		begin
  			prevIt := head;
  			head := head.next();
  