Versioning will be implemented as views.
We want to get statements that are 1) correct and 2) current. A statement (or all statements in a model) can be withdrawn.
A statement can be withdrawn because it has allways been false. See truth.
The second type of versioning says that some statement is true
during a specific time period. So let us specify that we want
to know the properties of a thing, as it was in
1999. $thing->arc( valid_during => $date_range
)->list. All this is dependent on that we decide on how
to describe dates, and more. The valid_during talks
about the arc and not the object. The valid_during can be
dynamicly infered from other
properties; for example from the model.
Not implemented