Exercise 1.4.11

PUSH and POP

The other half of the a stack involves destructively removing the first element from it. This can be done with pop.

* *stack*
(3 2 1)

* (pop *stack*)
3

* *stack*
(2 1)

* (pop *stack*)
2

* (pop *stack*)
1

* *stack*
NIL

Calling pop on an empty list has no effect.

* *stack*
NIL

* (pop *stack*)
NIL

* *stack*
NIL