Abstract supertype of datastructures that can be used as LIFO stacks.

A Stack has a well-defined top. Elements may be added to the top of the stack by push(), and removed from the top of the stack by pop().

Note that many Stacks are also Lists, but there is no defined relationship between the order of elements in the list and the direction of the stack. In particular, the top of the stack may be first element of the list, or it may be the last element of the list.