For example, I (ajr) am going to print test papers which have students' names pre-printed on the covers. This involves writing a little program. While the tests are printing, I'm going to be standing by the printer (for security!) and I will be examining the tests as they print, to make sure that everything is coming out right. I will be evaluating the correctness of my little program while more batches are still printing and more batches are yet to be printed. This is useful because if there is something wrong, I would like to catch it early and not actually go through with the rest of the printing until I fix my little program.