A stack implemented with an array. See [root:Stack_intf] for documentation.
The implementation will grow the array as necessary, and will never automatically
shrink the array. One can use set_capacity to explicitly resize the array.
capacity t returns the length of the array backing t.
set_capacity t capacity sets the length of the array backing t to max capacity
(length t). To shrink as much as possible, do set_capacity t 0.
set_capacity t capacity
set_capacity t 0