I can see (I think) how you transform

a a b
to
a a -
and

a a b
b b c
[download]

to
b b b
and

b b c
c d d
[download]

to - d d

but the rule I intuit for those doesn't work for the transformation of

b b c
c d d
[download]

to
c - c

CODE TAGS...

<c> ... data here
(even multiline data)... </c>

might help us to understand how you get from the source of the list to the output; that is, by what rule(s) are you doing the tranformations?

or is it just too late in the evening for me to understand?