Shallow Clone: A Shallow Clone is a clone that contains only nodes which are descendants of a single node, called its root or root node.
Wanted Nodes: The root's descendants are called wanted, all
Unwanted or Absent Nodes: All other than wanted are called unwanted or absent (to differentiate them from _missing_ ones,
which simply haven't been pulled yet).
A shallow clone is a Child of another if its root is contained in the other's wanted nodes.
The other is called the Parent.
Both are called Related.
Clones are called unrelated if their wanted node sets are disjunct.
They are called twins if their roots are equal.
For related clones, the child's root is called the Shallower root.
A revision is called Partial if some of its data has been reduced to accommodate partial history.
It is called Shallow if either of its parent references has been nulled.