public void emptyaction (Node n, Action a)
	// interpret a remove stone action
	{
		int i, j, r = 1;
		ListElement larg = a.arguments();
		while (larg != null)
		{
			String s = (String)larg.content();
			i = Field.i(s);
			j = Field.j(s);
			if (valid(i, j))
			{
				n.addchange(new Change(i, j, P.color(i, j), P.number(i, j)));
				if (P.color(i, j) < 0)
				{
					n.Pw++;
					Pw++;
				}
				else if (P.color(i, j) > 0)
				{
					n.Pb++;
					Pb++;
				}
				P.color(i, j, 0);
				update(i, j);
			}
			larg = larg.next();
		}
	}