This python code reads data from the file sqscell.out (generated by mcsqs code) and writes the first 3 cells with the smallest and equal vector lengths to a new file with the same name. The original data is saved in old-sqscell.out. The data in sqscell.out is in a structured format, with each cell represented by 4 lines: the first line indicates the total number of cells, followed by 3 lines of vector data. The code first reads the original file and saves a copy in old-sqscell.out. Then, it calculates the length of each vector and selects the first 3 cells with the smallest and equal vector lengths. Finally, the code writes the selected cells to a new sqscell.out file. This code helps to find cubic SQS supercells from all the supercells generated by mcsqs code under ATAT package.