vim mapping normal keys like shift

I want to map a key in Vim such that i would hold down a key, say a, and then press some other key, say b. And then the holding down of the a would change what the pressing of b does.

Like the way we use shift, just with all the normal characters of the keyboard.

I开发者_C百科s that possible? And if so, how?

The only way to do this would be to turn A into a normal modifier via xmodmap or the like, and the only modifiers vim supports are Ctrl, Alt, and Meta, so not only do you lose normal functionality of the key, but you also add it to a set which contains one or more keys already.