SECONDS-TO-NEXT-UPDATE handles variable args better.
Rather than just requiring a list, it can either execute its body or recurse.
Fix args to invocations of SECONDS-TO-NEXT-UPDATE.
Calling s-t-n-u with a list would return the next update of all the feeds.
Now it's more intuitive: given a list, it returns a list; atom for an atom.
No more reliance on semi-accidental behavior.