Implementation of n-ary sums and n-ary products.

The module Data.SOP is the main module of this library and contains more
detailed documentation.

The main use case of this package is to serve as the core of generics-sop.

A detailed description of the ideas behind this library is provided by the
paper:

  Edsko de Vries and Andres Loeh. True Sums of Products. Workshop on
  Generic Programming (WGP) 2014.
  http://www.andres-loeh.de/TrueSumsOfProducts