static <A> LL<A> groupInFixedSizeRows(L<A> l, int rowSize) {
  new LL<A> matrix;
  for (int i = 0; i < l(l); i += rowSize)
    matrix.add(subList(l, i, i+rowSize));
  ret matrix;
}

static <A> LL<A> groupInFixedSizeRows(A[] l, int rowSize) {
  ret groupInFixedSizeRows(asList(l), rowSize);
}