[If you have more time in this session, study the 
the two stack module alternatives (stack1/2), and the 
timing module (stacktime) in this directory.  A run of 
the test appears below; it demonstrates list operation 
efficiency issues, and timing tools we'll meet later.
What does it suggest about in-place operations (.append,
del) versus copy operations (slicing, concatenation)?]


% python stacktime.py
stack1: spam2499 spam2498
5.71
stack2: spam2499 spam2498
0.22