2.17 Assignment: set! and set!-values
(set! id expr)
Otherwise, evaluates expr and installs the result into the location for id, which must be bound as a local variable or defined as a top-level variable or module-level variable. If id refers to an imported binding, a syntax error is reported. If id refers to a top-level variable that has not been defined, the exn:fail:contract exception is raised.
See also compile-allow-set!-undefined.
(set!-values (id ...) expr)
More generally, the set!-values form is expanded to
(let-values ([(tmp-id ...) expr]) (set! id tmp-id) ...)
which triggers further expansion if any id has a transformer binding to an assignment transformer.