Anyone who had read the src of lalr-scm?

[康桥]

Hi all. I am a college student, recently I became interested in parser 
generating algorithms.
One of them is LALR(1).I took lalr-scm(which is mentioned in Guile Manual) as 
an example of the design of this kind of parser generators and studied its 
source code.
However, I found the source code too little scheme.It seems that it was written 
by a programmer who had been familier with C or Pascal before, mainly because 
it relies heaviliy on side-effects, which we don't use much in functional 
programming languages.
In spite of this, I read it almost all.I have found the part of computing 
itemsets, FOLLOW sets, etc.But I still leave one function not understand. It's 
function digraph in the macro lalr-parser in file lalr.scm.
It seems that this function is trying to traverse a digraph and make each 
FOLLOW set corresponding a vertex in the digraph the union of it and the FOLLOW 
sets corrsponding the vertices which can be reached from the vectex I mentioned 
before.But I can't figure out what algorithm it uses.

Anyone who know this algorithm?
BTW, should we rewrite lalr-scm in a more scheme way(even to make the process 
of parser generating slower)?