[luatex] Error: "! This can't happen (add_disc_widths)."

[Daniel Flipo]

Hi all,

In French typography, when two quotations are embedded, every line of 
the inner one should start with a quote character («) followed by a kern 
(there are variants, but let's stick to this one).

I am trying to implement this in LuaTeX using the following trick inside 
inner quotations:

-- When a line break occurs on a disc node, it is easy to add any hlist 
(« + kern in my case) at the beginning of the next line using the "post" 
field of the disc node; this part works fine.

-- As most line breaks occur on a glue node, I try to replace any glue 
node by a disc node with
"pre"     = empty,
"post"    = "« + kern",
"replace" = the replaced glue.

When I add the Lua code implementing this replacement (glue -> disc) to 
pre_linebreak_filter, and I get the error message:

"! This can't happen (add_disc_widths)."

Thanks in advance for comments and/or ideas for a workaround.

Best wishes,
--
Daniel Flipo