Keyboard remapping beyond Xmodmap and Xbd?

Hi,

I'd like to remap the keyboard so that each key behaves differently
when hit alone or along another key. For example, the space bar when
hit alone should send a space character, and should act as a modifier
if another key is hit before it is released, like this:

a => a
space + a => @
s => s
space + s => $
space => space

I know I should use keycodes, but the example above is for a simple
explanation of what I'm looking for.

Neither xmodmap nor xkb seem to allow this.

I've googled extensively and I think I'll have to write a driver and
recompile the kernel, but I'd like not to resort to that. I'd like
that to be a "portable" solution, so that I can save it onto a memory
stick and bring it along with me whenever I use another computer.

Thanks in advance.


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