sum-of-submodular
=================
sum-of-submodular implements algorithms for optimizing a sum of submodular 
functions. That is
    f(S) = sum_C f_C(S \cap C)
where the functions f_C are all submodular, and C are all subsets of a base set
V. Note that f itself is submodular, but if the f_C have nice structure, we 
can do better than general submodular optimization.
