public synchronized void white (int i, int j)
	// black move at i,j at the end of the main tree
	{
		if (i < 0 || j < 0 || i >= S || j >= S) return;
		TreeNode p = T.top();
		while (p.haschildren())
			p = p.firstChild();
		Action a = new Action("W", Field.string(i, j));
		Node n = new Node(p.node().number() + 1);
		n.addaction(a);
		p.addchild(new TreeNode(n));
		n.main(p);
		GF.yourMove(Pos != p);
		if (Pos == p) forward();
		MainColor = 1;
	}