  We give a 2-approximation algorithm for the Maximum Agreement Forest problem
on two rooted binary trees. This NP-hard problem has been studied extensively
in the past two decades, since it can be used to compute the Subtree
Prune-and-Regraft (SPR) distance between two phylogenetic trees. Our result
improves on the very recent 2.5-approximation algorithm due to Shi, Feng, You
and Wang (2015). Our algorithm is the first approximation algorithm for this
problem that uses LP duality in its analysis.
