My `additionalKeys` section of xmonad.hs has this key binding:
((mod3Mask, xK_d),               raiseMaybe (runInTerm "-title calendar -hold -geometry 105x12+0+25" "task calendar") (title =? "calendar"))

I would like to replace the raiseMaybe by something like runOrKill. This means, it should run the terminal with title "calendar" if it's not yet running. Otherwise it should kill the terminal. How can I do that? Is it possible at all?