[det]rdf_generation(-Generation)True when Generation is the current generation of the
database. Each modification to the database increments the generation.
It can be used to check the validity of cached results deduced from the
database. Committing a non-empty transaction increments the generation
When inside a transaction, Generation is unified to a term
TransactionStartGen + InsideTransactionGen. E.g., 4+3
means that the transaction was started at generation 4 of the global
database and we have created 3 new generations inside the transaction.
Note that this choice of representation allows for comparing generations
using Prolog arithmetic. Comparing a generation in one transaction with
a generation in another transaction is meaningless.