Контракты:
<tex> push:
void push(Object element) </tex>
<tex> pop:
pre: size > 0
post: size == size' - 1 && result == elements[size]
Object pop() </tex>
<tex> peek:
pre: size > 0
post result == elements[size - 1]
Object peek() </tex>
<tex> size:
post: result == size
int size()
isEmpty:
post: result == size > 0
boolean isEmpty() </tex>
