All my fault. Previously I had run software updater; then I intended to run
sudo apt-get update & sudo apt-get upgrade, but instead I accidentally
started software updater again, realized my error, and aborted software
updater. That action crossed up something in the retrieval of archived data.
I "fixed" the problem by changing a setting in software updater, running it
again, changing the setting back, and running software updater yet again.
This time it completed its cycle successfully without complaint, and now sudo
apt-get update works OK again.
Nothing to see here, folks; drive right on by and clear the traffic jam.
