To propagate down when the inheritance-method is depth-first:

Order the immediate parents, left to right.  The number each parent is
assigned is relative to the node, and it called the breadth-number.
Don't worry about parents of parents.

In breadth-first (first value), each slot is marked with 2 "digits", the
depth and the breadth number, respectively.  The breadth number is the same
here as it is with depth-first.
