Lab 10, Task 1
-------------
original stack:
+     +
| "f" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

calling pop() gives:
+     +
|     |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

calling push("x") gives:
+     +
| "x" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

calling push("w") gives:
+     +
| "w" |
| "x" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

s = pop() assigns "w" to s and gives:
+     +
| "x" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

calling pop() gives:
+     +
|     |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

calling push("z") gives:
+     +
| "z" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

because s is "w", calling push(s) gives:
+     +
| "w" |
| "z" |
| "e" |
| "a" |
| "c" |
| "b" |
+-----+

2. See StackClient.java

   We do not need a type cast for the following line:

       String str = s1.pop();

   because our Stack implementations are generic, and we specify that 
   s1 is a stack of Strings:

       Stack<String> s1 = new LLStack<String>();

   As a result, s1's version of pop() has a return type of String.

3. No, because we can only remove the topmost item in the stack, and 
   "z" is not the topmost item.