a tiny part of the lens library with no dependencies

 This is an extract from the lens package with no dependencies. It's not a toy
 lenses library, unsuitable for “real world”, but merely a small one. It is
 compatible with lens, and should have same performance. It also has better
 documentation.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.