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|+Rose (n-ary) trees with both upwards- (*i.e.* cached) and|
|+downwards-traveling (*i.e.* accumulating) monoidal annotations. This|
|+is used as the core data structure underlying the|
|+[diagrams framework](http://projects.haskell.org/diagrams), but|
|+potentially has other applications as well.|
|+Abstractly, a `DUALTree` is a rose (n-ary) tree with data of one type|
|+at leaves, data of another type at internal nodes, and two types of|
|+monoidal annotations, one travelling "up" the tree and|
|+one traveling "down".|
|+For a bit of background and motivation, see the paper|
|+[Brent Yorgey. *Monoids: Theme and Variations*.](http://www.cis.upenn.edu/~byorgey/pub/monoid-pearl.pdf) Haskell Symposium, 2012.|
|+ cabal install dual-tree|