Yes, it is fragile (less than five minutes, man!) but hardly unreadable to the macro-accustomed eye.

A "real" implementation would do more processing of the forms in the list read by read-delimited-list and perform appropriate checks. This is more to just give you a sense of how trivial it is to extend the language.

I don't actually know python well enough (I had to look up "list comprehension") to say, but I would estimate that it would only take a page or so of code to give you something fully-featured and robust.

In practice, I would never abuse the reader like this when typical "loop" expressions are enough to do these kind of things (and more).