review.

As before, I would suggest reviewing just the cython code and not the C code
since is generated, and hence hard to impossible to review.

while)

Cython can not handle negative indices ( because of a derictive at the
begining of the function that says not to do boundschecks). This is used to
make the code faster, so this workaround is better then removing the
directive.

scan)

All these changes have been generated by cython, and as such I don't think
they need to be reviewed but rather reviewing the cython code should be more
then sufficient.

In fact, it could hid problem that where not expected.

I will push a test for this latter today ..

If there is an error in the inner graph, without this change in the CVM you
do not get a message saying what was wrong.

graph

This fix a failing test.