16.2.3 Partitioning Trees

When regarded from a specific node, a tree is divided into 5 regions: (1) the node itself, (2) the nodes above, (3) the nodes below, (4) the nodes to the left, and (5) the nodes to the right.

We will aggregate the set of nodes to the left and to the right, and call the result the side set. A similar treatment can trivially be developed that retains the distinction; such a treatment would support precedence constraints.