Boxes

 

  MzScheme provides mutable boxes, (i.e., 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 example, (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?.