Thank you for the suggestions. I should have been using strict and warnings, of course. Maybe then I would have seen more easily where my problem was.

I don't have Conway's book, so I can't see in what context he recommends using the block (rather than the expression) form of map. But I do have reasons for wanting the expression form:

• It's legal syntax, so it should be useable. (And it is, of course, if you don't make the kind of mistake I did.)
• It's more elegant for a single-parameter function. At least I think so.