Return summed value of all descending spns
Examples
tree <- randTree(10)
getNdsPD(tree, ids = tree["all"]) # return PD of all ids
#> n1 n2 n3 n4 n5 n6 n7 n8
#> 7.9051891 3.9119778 2.9494452 3.2960982 1.4683960 1.2430801 0.3698908 0.8846579
#> n9 t1 t2 t3 t4 t5 t6 t7
#> 0.7169746 0.0000000 0.0000000 0.0000000 0.0000000 0.0000000 0.0000000 0.0000000
#> t8 t9 t10
#> 0.0000000 0.0000000 0.0000000