At the moment, GML, for Game Maker 7, because I'm helping out on a project.
I came here again just now actually to say that I found out.
I have a C++ for Dummies book laying around so I skimmed through it till I found the little bugger command.
+= at least in C++, and I'm guessing in GML too, does what I said it does.
Basically typeing
x += 2
is the exact same thing as typing
x = x + 2
.