how to modify `afterGraceFraction` mid-score?

[Werner LEMBERG]

In full scores it is sometimes necessary to adjust the value of
`afterGraceFraction` so that it harmonizes with other notes and
rhythms.  However, saying

```
afterGraceFraction = <value>
```

only works at top-level according to the documentation.  How can I
change this value mid-score?


    Werner