A linked list is a pair containing the first element of the sequence (in this case 1) and the rest of the sequence (in this case a representation of 2, 3, 4). The second element is also a linked list. The rest of the inner-most linked list containing only 4 is 'empty' , a value that represents an empty linked list.

Linked lists have recursive structure: the rest of a linked list is a linked list or 'empty' . We can define an abstract data representation to validate, construct, and select the components of linked lists.