replace defer() with a decorator?

Just an idea: would it be possible to replace defer()/defer_unless() with a method decorator? So something like:

```
def _get_foo(self):
    self.defer_unless( someproperty )
    # calculate...
```

becomes

```
@depends_variation( someproperty )
def _get_foo(self):
    # calculate...
```

It may even be possible to automatically trace dependencies.
