marker direction if average direction is zero?

Hello www-svg,

if a path itself has a directionality at the corners/vertices,
but the average is zero, what is the correct behaviour?
See attachment for examples.

I found a definition, if the path has no directionality, but not
about the problem, that the average direction can be a zero 
vector (the tangent vectors have an opposite direction).

Even worse, current viewers display it typically different from each other.
I'm not sure, what a useful behaviour could be, therefore
maybe the usual local x-axis should apply.
For the next SVG version it might be useful as well to have an
additional property to suppress markers for vertices with no
directionality or with an average resulting in a zero vector.

Results with the usual suspects Opera, Batik/Squiggle, adobe-plugin, Gecko, 
WebKit, Konqueror+KSVG look
like random choice, result in Inkscape (0.46) is
completely nonsense.

Well, even for cases with a defined direction, the results are often quite
different for non-trivial samples. 
I think, currently the adobe plugin provides results almost as specified in
the recommendation. the Geckos quite often provide a correct
result. Others are more often wrong, therefore results from these
viewers are anyway no reliable source for a direction of a path as
specified ;o)


Olaf