unfolding of the 2D number correlation to extract net-distribution : N(A)-N(B)
- "incremental + conventional" hybrid approach 
- including volume fluctuation as a part of unfolding
- shown and discussed in BES workshop in Fudan University, Shanghai, China, 16/Aug/2017
- discussed in EMMI workshop in CCNU, Wuham, China, 10/Oct/2017