Ah, I read too quickly before.  But that expression "when a delimiter string *sep* is given" is hard to wrap ones head around in this context.  I think the problem really is that 'split' has such radically different behavior when given an argument as opposed to when it isn't.  I consider that a design flaw in split, and always have.  So, I suppose we can't do any better here because of that.

Please move the keeplines discussion back up into the initial paragraph, and then I think we'll be good to go.