It was originally `class:` (that is now squashed out of commit history), but I found that when writing examples I wrote `class` by mistake and got very confusing errors all the time.

It may also be worth considering changing `struct:` to `struct` and `for:` to `for`, etc. too. The typed versions of these shouldn't be ambiguous with the untyped grammars.

OTOH, if the consensus is that this is a bad idea I can change it to `class:`. But in that case, we should put more effort into making the non `:` versions produce more useful error messages.