How to translate input command in comint before send to debugger?

[Z]

Hi all,
I use pdb to debug Python programs. But pdb is lack of command
like "u 3"(means up 3 frames). So I'd like to translate "u 3" to
"u;;u;;u" in comint before it is send to external debugger.

I have tried to add hook to comint-input-filter-functions, but it seems only 
receive the input and don't return translated command
to commit. Any suggestion to do that?

Thank you very much,
Z