public void placeaction (Node n, Action a, int c)
	// interpret a set move action, update the last move marker,
	// c being the color of the move.
	{
		int i, j;
		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)));
				P.color(i, j, c);
				update(i, j);
			}
			larg = larg.next();
		}
	}