Figure 3
From: Local search for the generalized tree alignment problem
Three directional assignment. Three possible assignments to interior vertices of an unrooted tree. Left: computing the subdivision vertex of (s,v), or any edge rooted by s (grey triangle on s), would require to compute the assignment to v using those of t and r. Center and right: similarly, the assignment of v could be computed using s and t, or t and r. Each direction is needed for some subdivision vertices.