Minor optimization + comments

main.cpp

@@ -104,6 +104,7 @@ public:
         std::make_heap(heap.begin(), heap.end(), sort_cells);
     }
 
+    // TODO: Turn this into a member of Cell as opterator<
     static bool sort_cells(const Cell& a, const Cell& b) {
         return a.possible_values.size() < b.possible_values.size(); 
     }