SpiceDB is an open source, Google Zanzibar-inspired, database system for creating and managing security-critical application permissions.

SpiceDB

In the following example:
```definition user {}

definition folder {
    relation reader: user
    permission read = reader
}

definition document {
    relation parent: folder

    permission read = parent->read
}```
lets say that I want users to have `read` permissions whenever a document does *not* have a parent_folder. What would be a nice way to go about this? I guess this is another instance where the intersection arrow would come in handy.

A possible solution could be:
```definition user {}

definition folder {
    relation reader: user
    permission read = reader
}

definition document {
    relation parent: folder | folderless

    permission read = parent->read
}

definition folderless {
    relation reader: user:*
    permission read = reader
}```
but it feels a bit off, since there is no way to guarantee folder and folderless are mutually exclusive. Does anyone have some tips or ideas on how to solve this?