Thanks to everyone for their interest in this topic. In reference to https://github.com/quicklisp/quicklisp-client/pull/128/, it is only a minimal stop-gap measure which I found could fix the immediate problem which I encountered. I know very little about ASDF and Quicklisp, so I certainly had no conscious intent to assert that one condition or another must prevail. Any improvements by more knowledgeable people would be welcomed by me.
All the best, Robert Dodier