85b7463 change def of reducer to take function ptr fe6366e add engine base a98720e more deps git-subtree-dir: subtree/rabit git-subtree-split: 85b746394e0bed36a22ebb12beb8672616b39047
git-subtree-dir: subtree/rabit git-subtree-split: c7282acb2a92e1d6a32614889ff466fec58937f3