Naming of data frame inputs

[gadenbuie]

I've been using x and y for the input data frames and z to reference the diff between the two.

On the one hand, I like it because it's agnostic to the actual work flow of the user but x does receive slight though minimal preference.

Another thought would be to use left vs right or reference vs comparison. I think these are overly wordy, but they may be more approachable to a new user.

ATM, I'm moving towards x,y,z as the standard but this can change depending on input or other considerations.