Back in high school I was on the US FIRST robotics team, and a student came up with a transmission design that appears to be effectively the same as what's described here. It consisted of two input motors, whose relative speed determined the output speed. The main motor was attached to the sun gear of a planetary gear system. Around the sun, there were three planetary gears that were attached to the output shaft. Around the planetary gears, there was a outer ring gear, which also had teeth on the outside that were attached to the secondary motor. Thus, by varying the relative speed of the input motors, you could get the system of planetary gears to be stationary, or to rotate at variable speeds in either direction. This was back in 2003.

I've memorized Pi exactly. It's just 10, but that's in base Pi of course.