Move sum to its own file. [1.31]
Add product function. [1.31]
Define factorial in terms of product. [1.31]
Define Wallis's method for pi. [1.31]
Recursive implementation of product. [1.31]
Merge branch 'ex1.31'
Redefine sum and product as accumulate. [1.32]
Recursive accumulate. [1.32]
Merge branch 'ex1.32'