Hello, In the example, is file1 like this ? : line1 line2 line3 line4 line5 line1 line2 line3 line5 line0 And file2 like this ? : line1 line3 line5 line0 line1 line3 line4 line5 I am not sure that I have understood how these files are made of. Or pear-heaps is it like that ? file1: file2: line1 line1 line2 other3 line3 line3 line4 other4 line5 line5 other1 line0 line1 line1 line2 other5 line3 line3 other2 line4 line5 line5 line0 other6 But in the second hypothesis, line2 doesn't exist in file2.