Figure 4

Constraint tree. Constraint tree, labels are omitted. Subtree 2 is coloured green. It has two child nodes that are leaves, therefore, n ₂ =2. The ancestor function for this tree is defined as f(2)=f(3)=1 and f(4)=2. A compact notation for this constraint tree is $(n_1,\dots ,n_k,f)=(0,2,3,2,\{(2,1),(3,1),(4,2)\})$.