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+[![Build Status](https://secure.travis-ci.org/diagrams/dual-tree.png)](http://travis-ci.org/diagrams/dual-tree)
+
+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.
+
+To install,
+
+    cabal install dual-tree