1. For every node of the graph and every URI reference used as a property or datatype at least one of the following holds:
  2. No blank node occurs as the object of more than one triple.
  3. No directed cycle of blank nodes occurs.
  4. Every blank node with explicit type rdf:List is the object of exactly one triple.
  5. Every builtin uriref is assigned to one of its listed categories, or to itself.
  6. Every other uriref is assigned to one of the following categories: