Selection of partial column subsets on a partition tree. Toy example (continued) - Selection of nodes corresponding to partial columns using the partition tree. The partition tree of the example in Figure 1 features as green nodes the "highest" non-ambiguous nodes. The leaves are the sites (i, p) in the sequences i at position p (note that numbering starts with 0). Sets of leaves sitting underneath a green node correspond to selected partial alignment columns. An inner node is denoted by a suffixed letter, e.g., A2-4 corresponds to an equivalence class for N = 4. The green nodes share the property of being non-ambiguous and having an ambiguous node as direct ancestor.