[Dev-luatex] order of node linking matters

[Paweł Jackowski]

Dear Colleagues,

the following code fails assertion (luafont.c, line 1950). But it is
enough to transpone last two lines of lua code to avoid it. Could 
someone explain why the order of node linking matters here?

\nokerns=1 \noligs=1
\setbox100=\hbox{ab\-xx}
\directlua{% here we move glyph b into disc node
	local a = tex.box[100].list % glyph a
	local b = a.next            % glyph b
	local d = b.next            % discretionary
	local _ = d.pre             % prebreak dash

	a.next = d  d.prev = a      % couple glyph a with disc
	d.pre = b                   % let glyph b be prebreak
	b.next = _  _.prev = b      % couple glyph b with dash
}
\hsize0pt \overfullrule0pt \hfuzz\maxdimen % force hyphenation
\unhbox100                                 % flush the list
\end

Apparently the assigment

	discretionary.pre = someglyph
	someglyph.next = someother

and

	someglyph.next = someother
	discretionary.pre = someglyph

is not the same. Why?

Warm greets to u all, Paweł
-- 
Pawe/l Jackowski
P.Jackowski at gust.org.pl