How to extract the scheme parser from the source?

Hi everyone!

I'm trying to implement a enhanced r5rs scheme system. Cause the 
enhancement is small, say, only a new data type and some operations on 
it will be added, working on a existing scheme implementation fits me 
better than working from scratch and I've chosen scheme48 as my basis.

The new datatype will bring new syntax, so the first step is making the 
scheme48's parser recognize the new syntax. I have not read through the 
source code of scheme48 before and I want a quick start.

My questions are:
1. Is scheme48's scheme parser written in Scheme or C?
2. Which file in the source tree should I concentrate on if I want to 
add a new syntax?

Any help is appreciated!