* function: next_slide to extend a linked list of type slide by one element
* function: new_line to initialize a new linked list of type line
* function: next_line to extend a linked list of type line by one element
* function: next_slide to extend a linked list of type slide by one element
* function: new_line to initialize a new linked list of type line
* function: next_line to extend a linked list of type line by one element