I think the `!` part might be a typo, and that pseudocode needs more context to be valid, but the individual elements are themselves valid even if you'd never see them composed in this way, and I wouldn't expect any regular user to have to ever read or write anything like this. The leading `for<>` indicates a higher-ranked lifetime (which I've never once needed in eight years of writing Rust (though to be fair, I think it's only existed for like six of those years...), the `!` is the "Never" type indicating a statically-impossible state (honestly it really should just be called `Never` rather than `!`, this is a leftover artifact of prehistoric versions of Rust that predate even me. Maybe I'll write an RFC to propose that), the `?Sized` is a one-of-a-kind trait bound that users might need rarely (but I don't bother with, generally), and the `'a` is a lifetime type parameter (which is common for anyone working with references, although it's such an important part of the language that you don't bat an eye at it after a while).