That solves my problem and helps me to understand how the set origin feature works. I had tried to use the tool to establish the point at which x,y, and z axis values are zero. It didn't work as I thought it should.
As I understand it now, the tool allows the user to choose one plane out of the set of all planes parallel to the current workplane. How about "Set Workplane level" as opposed to "Set Workplane Origin?"
Perhaps my understanding is still incomplete. If so I sit ready to be corrected!