SRFI-111 Boxes.

SRFI-111 provides boxes: objects with a single mutable cell.

Scheme Procedure: box value

Return a newly allocated box whose contents is initialized to value.

Scheme Procedure: box? obj

Return true if obj is a box, otherwise return false.

Scheme Procedure: unbox box

Return the current contents of box.

Scheme Procedure: set-box! box value

Set the contents of box to value.