Piecewise fix for 0d

I provide a fix for #331. Actually little changes were needed, but I had to carefully think about the assumptions being made by the function. Some additional tests were added and the behavior remains backwards compatible.

Besides, I corrected the documentation because there was an inconsistent bit, see 0d1f7d3.
yes nan would make more sense but we want to release 1.9 soon and I don't want to rush in a change that might bite us later
leaving the status quo of undefined is safer, we can revisit for 1.10