You can see this in the wild in 'parseurl in srv.arc:
(let (type url) (tokens s) ...)
Here are two examples destructuring a 3-element list into a 2-element arg, and into a 4-element arg:
arc> (let (x y) '(a b c) y) b ; no error arc> (let (x y z nothing) '(a b c) nothing) nil ; no error