Feature request: constrain the prefix of a group

[DJWassink]

The newly implemented where method is great! But we can do beter 🌮 .

The group method is such an awesome feature which can be made even more 🌮 by allowing the where method to also work on the group!

[JBlaak]

We should first do a refactoring round before diving into this, but this is definitively something we "need"