The two functions RemHead() and RemTail() are used in combination with
AddHead() and AddTail() to create special list ordering.  When you combine
AddTail() and RemHead(), you produce a first-in-first-out (FIFO) list.
When you combine AddHead() and RemHead() a last-in-first-out (LIFO or
stack) list is produced.  RemTail() exists for symmetry. Other
combinations of these functions can also be used productively.

Both RemHead() and RemTail() remove a node from the list, and return a
pointer to the removed node. If the list is empty, the function return a
NULL result.