Needs a bit of investigation - this might be better implemented through checks in validator tools like the CRS4 validator.
#211 (comment)
"all entities should be directly or indirectly reachable from the Root Data Entity via cross-references": currently this is automatically true for data entities (see above), but not for contextual entities. I'm not sure we should even check, since this is not a validator, especially if it turns out to be computationally expensive. Possibly another low priority issue.