I have mixed feelings on whether that case should fail, and it depends on whether we are conforming the entire message or the properties inside the message. If the former, the "mixed" case should fail. If the latter, the "mixed" case should succeed. There may be more complicated cases in the future when it comes to more advanced `conform!` methods.

I'm also wondering whether these `conform!` methods should be set up to allow some sort of chaining, which would capture the data flow from one format to the next.