Boxes in The Racket Guide introduces boxes.
A box is like a single-element vector, normally used as minimal mutable storage.
A literal or printed box starts with #&. See Reading Boxes for information on reading boxes and Printing Boxes for information on printing boxes.
Returns #t if v is a box, #f otherwise.
Returns a new mutable box that contains v.
(box-immutable v) → (and/c box? immutable?) v : any/c
Returns a new immutable box that contains v.
Returns the content of box.
For any v, (unbox (box v)) returns v.
Sets the content of box to v.