// TODO: all the fringe cases (?)
static <A> JPanel hvgrid(L<L<A>> components) {
  if (empty(components)) ret new JPanel;
  int h = l(components), w = l(first(components));
  new JPanel panel;
  panel.setLayout(new GridLayout(h, w));
  for (L<A> row : components)
    smartAdd(panel, row);
  ret panel;  
}

static <A> JPanel hvgrid(L<L<A>> components, int gap) {
  JPanel panel = hvgrid(components);
  GridLayout g = cast panel.getLayout();
  g.setHgap(gap);
  g.setVgap(gap);
  ret panel;
}