ms609

     poEdge <- Postorder(tree())$edge
     parent <- poEdge[, 1]
     child <- poEdge[, 2]
     xy <- matrix(0, parent[1], 2)
     xy[seq_len(NTip(tree())), ] <- mapping()
     for (node in unique(parent)) {
       xy[node, ] <- colMeans(xy[child[parent == node], , drop = FALSE])
     }
     segments(xy[parent, 1], xy[parent, 2],