$func Box e.Exp = s.Box;
$func Get s.Box = e.Exp;
$func Store s.Box e.Exp = ;
Box creates a new box, puts the expression e.Exp into the
box, and returns a reference to the box.
Get returns the contents of the box referred to by s.Box.
Store puts the expression e.Box into the box referred to
by s.Box.