Resolves #785

I think #749 is worth looking at too perhaps (mostly needs changing in dom-indexed). As much as I dislike the idea of stringly typing, the context-based behaviour of the values that can be provided here suggests that maybe a more precise modelling isn't much use anyway. I don't know... it might still be nice to enumerate the many various options in a type anyway, since at least then it prevents typos / allows discovery by editor autocomplete, etc. What do you think @thomashoneyman?