Boxes

MzScheme provides boxes, records with a single mutable field:

• (box v) returns a new box that contains v.
• (unbox b) returns the content of the box b. For any v, (unbox (box v)) returns v.
• (set-box! b v) sets the content of the box b to v.
• (box? b) returns #t if b is a box, #f otherwise.

Two boxes are equal? if the contents of the boxes are equal?.