[ii41]

This is equivalent to the `if let` syntax of Rust. In Rust at least this is treated as a special syntax but not a boolean. I once complained it's kinda confusing that it makes one think the `let` pattern matching syntax is a boolean expression which it is not, and the Rust people replied to me saying yeah it can be a boolean maybe at some point in the future. So yeah whether such syntax makes pattern matching a boolean really is just a matter of whether it's said to be one.