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Forget what I said in the last comment.  I must have been
asleep.

The problem boils down to this:  OS X does lazy memory
allocation and each process gets up to 4GB of virtual memory.

I now know a lot more about how the interpreter manages
memory.  I could have saved myself the trouble by paying
attention to what the VM: 4.19G+ in top was telling me.

The test in test_b1.py should not be run on OS X.  I am
going to avoid problems like this in the future by setting a
hard limit on per process memory of 400MB.