Behaviour of dx(0) , ds(0), dS(0)

Hi everybody!
Does anybody know precisely what is the behaviour of the measures dx(0) , ds(0), dS(0) in the last version of Fenics?
If I remember correctly in earlier versions at least ds(0) was somehow restricted, so I just avoided marking subdomains and boundary subdomains using ‘0’ and starting from ‘1’ instead.
Is this still the case? It would be nice to start numbering subdomains from 0 to match python’s array numbering scheme and avoid potential mistakes.
Thanks for your help!

You can start from 0, there is no restriction, But be aware that many people initialize arrays with Zero, and then you risk getting Extra contributions of you are not careful.