public void setaction (Node n, Action a, int c)
	// interpret a set move action, update the last move marker,
	// c being the color of the move.
	{
		String s = (String)a.arguments().content();
		int i = Field.i(s);
		int j = Field.j(s);
		if ( !valid(i, j)) return;
		n.addchange(new Change(i, j, P.color(i, j), P.number(i, j)));
		P.color(i, j, c);
		P.number(i, j, n.number() - 1);
		showlast = false;
		update(lasti, lastj);
		showlast = true;
		lasti = i;
		lastj = j;
		update(i, j);
		P.color( -c);
		capture(i, j, n);
	}