Build submodel on top of existed model.
Move root node out of constructor.
Move root node out of constructor.
Reuse fix decomposition tree and mp model APIs.
Fix bug: the variable cannot be registered by the new name once the name was changed.
Return None as default decomposition tree.
Add max cliques decomposer. See <http://networkx.lanl.gov/reference/generated/networkx.algorithms.clique.find_cliques.html>.
Fix logging message.