require: /** * before: * file A1.csv {1,2,3,4,5} * file A2.csv {2,3,9,10,11} * file B1.csv {5,12,13,14,15} * file B2.csv {16,14,15,4,9,20,30} * A1.csv A2.csv A3.csv A4.csv cant not repeat * * after: * file A1.csv {1,4} * file A2.csv {2,3,10,11} * fil