/write -l0 will overwrite the first line of a file. This is disastrous if someone uses $readn to find and delete or overwrite a line without checking to see if $read succeeded.
Plus, it doesn't make sense to delete the 0th line.
The point he's making is that people might use $readn in a script to overwrite a line just read, and then if the read fails they'll end up overwriting the first line.
I would expect -l0 to replace the first line only if it's a number (which represents the amount of lines), seeing how $read(file,0) works exactly in the same way.
