This program illustrates the "LIFO" (Last In, First Out) behavior of a simple
stack of characters, as well as its default constructor, its copy constructor,
and the stack push(), pop(), top(), empty(), and size() member functions.
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The stack s1 contains 4 values.
The stack s2 is created as a copy of s1, after which another value is added,
so its size is 5.
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Here the values of s1, in "LIFO" order:
Popping: D
Popping: C
Popping: B
Popping: A
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Here the values of s2, in "LIFO" order:
Popping: E
Popping: D
Popping: C
Popping: B
Popping: A
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