Thanks for the report, but this shouldn't be considered a bug. That unit is defined so that you can write "1 gram energy" or the like to find the energy content in that particular amount of mass. It's actually shorter (on a reasonable keyboard) to type "c^2" though, so this may not really be necessary. I may remove it from the data file.

That makes sense. I actually like the way that reads, and would suggest keeping it and adding sample usage to make it absolutely clear to people.