Feedback on row-reduction

[danmargalit]

In reference to version 224734d7be7ff0c4a8adb3a206356ea122112f33

In reference to version 224734d7be7ff0c4a8adb3a206356ea122112f33

Delete the third part of the defn of REF. Can comment afterward that this follows, meaning that those variables don't appear in later equations.