type Cell<x> (Cell( x[] ));

cell := λ(: x x). (: (
   (let c (Cell( (close x) )))
   c
) Cell<x>);

deref := λ(: c Cell<x>). (: (
   (let x (open(.1(as c Tag<Cell>))))
   x
) x);

set[] := λ(: c Cell<x>)(: x x). (: (
   (set[]( (.1(as c Tag<Cell>)) x ))
) Nil);