minor comment/code changes for clarity
committing before Expander rewrite
Expander expands summations raised to integer powers.

Corrected test case evaluation which caused tests to pass incorrectly.
    PROBLEM: Each test case was "expand(<expression>)".
      I parsed the test case, producing a tree with ExpandOp at the root.
      Then I called Expander().expand(tree), which left the ExpandOp
      in the tree and expanded the children (but not fully). Then I called
      reduce on the result, which *also* expanded the result since
      the ExpandOp was still in the tree. This resulted in test
      cases passing that should have failed.
    FIX: No longer call reduce on the test case.

    I reworked through the expansion algorithm and added a few test
    cases so I believe it's fixed (but it's still slow).