I faced a troublesome problem in my programs: I have made a lot of different versions of the same program with just a little difference between. This difference can result in a complete different performance and cannot be ignored. However, I cannot remember which one is which and what are their dissimilarities. In order to find out this, it is necessary to compare two different files and to do so, I open two codes in two different windows and switch between them very fast (a few many times a second). Human vision (like most other animals including lizards (remember Jurassic park!) is very sensitive to changes, thus I can detect a single character difference between two pages very easily. Anyway, this can be applied whenever there is no tremendous difference between those two files.