Fix the up/down method call on Diagram

After updating from the latest commit, I got many errors (even on the example and generator pages). It seems that I was a few version behind and still had the "old" version of up/down reduce in Diagram. The new version is lacking the first parameter from apply*, resulting with a *-Infinity* for *up* and *down*. I hesitated between *null* and *this*, but I can't figure a case where *this* would be required here.