public void addBlock( int x, int y, Block b ) {
			int index = stackIndex(x,y);
			Block[] stack = blockStacks[index];
			if( stack == null || stack.length == 0 ) {
				blockStacks[index] = b.stack;
				return;
			}
			for( int j=0; j<stack.length; ++j ) {
				if( stack[j] == b ) return;
			}
			Block[] newStack = new Block[stack.length+1];
			for( int j=0; j<stack.length; ++j ) newStack[j] = stack[j];
			newStack[stack.length] = b;
			blockStacks[index] = newStack;
		}