Catching every key typed into a window

[Rafal Kolanski <rafalk cse unsw edu au>]

Greetings Gentlemen,

I am looking for a way to inspect keys typed into a window and passing 
them to another function for processing, allowing maneuvers like 
replacing a sequence of chars with another, or noticing what's just been 
typed and completing the next bit automatically in uncooperative apps. I 
could do a bind for every single key to call a command, but that's very 
inflexible.

I'm looking at the docs for unbound-key-hook, which looks like it should 
be what I want, but I have no clue how to use it properly without 
rendering my keyboard useless. From the docs, I'm guessing I should 
return nil. Any advice?

Sincerely,

Rafal Kolanski.