Write subsets. [2.32]
Merge branch 'ex2.32'
accumulate function from book. [2.33]
Implement map. [2.33]
Implement append. [2.33]
Implement length. [2.33]
Merge branch 'ex2.33'
Break file into two.
Horner skeleton from book. [2.34]
Write Horner evaluator. [2.34]
Merge branch 'ex2.34'
Write count-leaves, with a lot of help from Bill the Lizard. [2.35]
Merge branch 'ex2.35'