page.data.type === "art-item". This way, I don't even have to list all the fields a template accepts, just the type is enough. 
I finally solved the last big problem with my SSG: handling page data. The way it works now is that you tell Corvos in the config what data pages can have, and if you choose a field as type discriminator, you can easily get the full page data by just doing e.g. 