typedef struct _Node {
    void* a_value;  // This is the same spirit as with qsort(…) in HW13.
    struct _Node* next;
} Node;

void append(void* a_value, Node** a_head, Node** a_tail);
void print_list(Node* head, void(*print_one_element_fn)(void*));
void free_list(Node** a_head, Node** a_tail, void(*free_value_fn)(void*));

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