10-42?! Nice Mark.
I run a fairly ridiculous combination of Campy shifters and front derailleur with Shimano mtb rapid rise rear derailleur. I do so for the aforementioned reason of needing a larger cassette capacity than campy rear derailleurs can handle. And for what it's worth, the back-asswards shifting of the rapid-rise low-normal rear derailleur is brilliant for cyclocross type riding in the trails because if I come around a corner to find a sudden uphill, I can hit the lever on the shifter down and jump up like four cogs in one motion and jam up the hill! Much faster than having to hit the lever inwards multiple times to do the same shift if I had a normal derailleur.
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Thirst is stronger than the rules. - Stars and Watercarriers, 1974