public static Vector Rrowsum_matrix(Vector x, int ncol, AtomicVector groups, AtomicVector ugroup, boolean naRm) {
int numGroups = ugroup.length();
Matrix source = new Matrix(x, ncol);
MatrixBuilder result = source.newBuilder(numGroups, ncol);
for(int col=0;col!=ncol;++col) {
// sum the rows in this column by group