Require positive domain id

[Prof Garth Wells]

Integrals can have an integer associated with them, e.g. dx(1), to indicate a subdomain. It would be helpful if this was required to be a non-negative integer, as behind-the-scenes we could reserve -1 for dx.

Perhaps even simpler would be requiring the the integer be strictly positive, in which case we could reserve 0 for dx.

[Chris Richardson]

I think we can easily enforce strict positive, as nobody would be using negative numbers at the moment (as far as I know).

[Prof Garth Wells]

@chris_richardson People may well be using dx(0), disallowing this would potentially break code.

[Chris Richardson]

Sure, I mis-spoke as they say. I meant: non-negative.