**Optimization substep**

This substep begins with the completion of the growth substep - which has given us an extended leaf (by adding new cells at the fringe) along with a transport system for the extended leaf.

Now what we do is to look at the cells, one a time, and see if the total transport cost can be reduced by changing its parent to some other cell that is nearby to it. If so, then the transport graph is changed to replace its parent with the new parent.

This process is done repeatedly, until there are no further reparenting operations that can reduce the total cost of the transport system.

The resulting transport system is optimized as well as can be done -- without global restructuring.