Recursive update exception: assignment to a cage with lazy right hand side is a problem!

```
[] > infinite-loop
  cage > x
  seq > @
    x.write 0
    x.write (x.add 1)
    x.eq 1
```
fails. 

Of course: `x.write (x.add 1)` is implemented basically as `x := (lambda nothing -> x + 1)`. Any attempt to calculate `x` leads to endless recursion. 
