x1 (node in a tree graph) and x2 (node in the same tree graph) have an essentially unique most recent (graph-nearest) common ancestor node A such that x3 [nonnegative integer; li] is the minimum element of the set consisting only of d(A, x1) and of d(A, x2), and such that x4 [integer; li] is d(A, x1) - d(A, x2), where d is the graph geodesic distance (defined to be infinite if nodes are not connected in the correct direction).