i wonder if this is the right place to do the validation. i feel the best place might be the ID generators. with the assigned id stragety, you might do the same problem with `/my/path|*` or something. but with assigned id, the `/` is ok. in the id strategy, we know what we look at and which pieces we take to build the id.

the id strategy is also used later, when moving a document by assigning a different name for example.

@sarcher care to try doing it there? the strategies live in the sub-namespace ID. for the $parent we need no validation, only for the full path in AssignedIdGenerator and for the name in ParentIdGenerator.