Known-folded permissions not working

The following program should verify thanks to known-folded permissions, but does not:

```
field val: Int

predicate knownFolded(x: Ref) {
    acc(x.val, 1/2)
}

method test(x: Ref, y: Ref)
    requires acc(x.val, 1/2) && x.val == 5
    requires acc(y.val)
{
    fold knownFolded(x)
    exhale acc(y.val)
    inhale acc(x.val, 1/2)
    assert x.val == 5
}
```