Single_Linked_List := Object clone do(
    head ::= nil;
 
  // constructor
  init := method(
    head = nil
  );
 
  // methods
  isEmpty := method(
    self head == nil
  );
 
  empty := method(
	self head = nil
	);
 
  insert_at_tail := method(n,
    if(self head == nil,
      self setHead(Linked_List_Node clone)
    );
    self head insert_at_tail(n);
 
  );
 
  insert_at_head := method(n,
	newnode := Linked_List_Node clone;
	if(self head == nil, 
		self setHead(newnode),
		newnode link = self head;
		self setHead(newnode)
	);		
	self head insert_at_head(n);
  );
 
  remove_at_head := method(
    if(self head == nil, return nil);
    if(self head link == nil, self head value println; self setHead(nil), self setHead(self head link));
 
    );
 
  )
)
 
Linked_List_Node := Object clone do(
  value ::= nil;
  link ::= nil;
 
  // constructor
  init := method(
    value = nil;
    link = nil
  );
 
  // methods
  insert_at_tail := method(n,
    if(self value == nil,
      self setValue(n),
      if (self link == nil,
        self setLink(Linked_List_Node clone)
      );
      self link insert_at_tail(n)
    );
 
  );
 
  insert_at_head := method(n,
	self setValue(n));
 
  push := method(n,
	self setValue(n);
 
	);
 
  enqueue := method(n,
	self setValue(n));
)
 
Stack_2 := Object Single_Linked_List clone do(
    head ::= nil;
 
  // constructor
  init := method(
    head = nil
  );
 
  // methods
  isEmpty := method(
    self head == nil
  );
 
  push := method(n,
      newhead := Linked_List_Node clone;
	  if(self head == nil, newhead link = nil, newhead link = self head
    );
      self setHead(newhead);	  
      self head push(n);
 
  );
 
  pop := method(
    if(self head == nil, return nil);
	if(self head link == nil, self head value println, self head value println; setHead(self head link)   
    );
 
  )
)
 
Queue_2 := Single_Linked_List clone do(
    head ::= nil;
	tail ::= nil;
 
  // constructor
  init := method(
    head = nil;
	tail = nil
  );
 
  // methods
 
  enqueue := method(n,
      newnode := Linked_List_Node clone;
	  if(self tail == nil, self setHead(newnode); self setTail(newnode); newnode link = nil, self tail link = newnode; self setTail(newnode)
 
    );        
      self tail enqueue(n);
 
  );
 
  dequeue := method(
    if(self head == nil, return nil, self head value println; setHead(self head link));	
 
  )
)