In Odoo 12 and earlier it was possible to setup a "dummy" parent company so that customers / suppliers and products could be shared across several companies, but not all companies, so for example: 

• Companies A, B & C share customers / suppliers and products
  

• Company D does not share

• Company X is a dummy parent company for Companies A, B & C

In Odoo 13 users can access multiple companies (so in the above example, users could have companies A, B & C all active).  Therefore the Record Rule no longer references the parent and child companies.

But I think we have lost the "dummy parent company" functionality.  Can anyone suggest the record rule that would be required to add that back?