docs: public int growSuperNode() {
      if(getNumEntries() < getCapacity()) {
        throw new IllegalStateException("This node is not yet overflowing (only " + getNumEntries() + " of " + getCapacity() + " entries)");
      }
      Entry[] old_nodes = super.entries.clone();
      assert old_nodes[old_nodes.length - 1] != null;
      super.entries = (Entry[]) java.util.Arrays.copyOfRange(old_nodes, 0, getCapacity() * 2 - 1, entries.getClass());
      assert super.entries.length == old_nodes.length * 2 - 1;
      return getCapacity();
    }