FROM QUESTIONS AFTER CLASS...
=============================

Suppose we start with:

head
↓
[10|→] → NULL

Then we add a new node with the value 9.

    int* a_new_value = malloc(sizeof(*a_new_value));
    *a_new_value = 9;
    pq_enqueue(&head, a_new_value, cmp_fn);

The new node will become the new head of the list.  To do that, we will write
the address of the new node at address head.

head
↓
[9|→] → [10|→] → NULL

Then, we add a new node with the value 11.

    a_new_value = malloc(sizeof(*a_new_value));
    *a_new_value = 11;
    pq_enqueue(&head, a_new_value, cmp_fn);

head
↓
[9|→] → [10|→] → [11|→] → NULL