Variant positions less than one?

[hyanwong]

I'm assuming that variant positions must be 1 or greater in an sgkit dataset. Is this part of the spec, or enforced anywhere? I looked through the docs and couldn't find anything. I assume setting position to -1 will also break stuff?

In tskit the positions start at 0, so there's a bit of trickery needed in my use-case, I suspect.