When using the verse package in Latex, remember that if you put a square bracket right after a \\[ret] (which appears at the end of a verse line except the last), you will get an error. To get around that error, put the \null command just before the square bracket. I have no idea why it does that or why it works with the no-op \null there but it does.
\\[length] is the command to break line with added spacing. My guess is that whitespace isn't significant in that bit of TeX's parser, so you need the \null to stop it from eating the [ and looking for a length.
Interesting, the verse package supposedly redefines \\ to mean end of verse line. It is probably as you say it. There are some days when I want to show my Humanities colleagues Latex but there are days like today that make me hesitate.
