**2.2 Optimal transport systems**
Suppose we are given a "candidate leaf" consisting of some set of cells in the grid. A real candidate leaf, having been grown from the starting state with just one cell at the root, will be a connected set of cells.
However, for present purposes, let's forget about that, and consider any set of cells which contains the root cell as a candidate leaf.
We would now like to consider various possible piping systems for this collection of cells - and to seek ones that have minimal cost.