diagrams/dual-tree — pull request #5 by byorgey (2013-11-05 21:49:50+00:00)
Title: Expose internal d nodes via foldDUAL

Previously, we prudishly declared that users must not be given access to internal `d` annotations. However, this turns out to be precisely what we need to implement backends via trees. This pull request changes `foldDUAL` to give access to internal `d` nodes, and adds more commentary explaining its correct use.