public ColumnArrangement(HorizontalAlignment hAlign, 
                             VerticalAlignment vAlign,
                             double hGap, double vGap) {        
        this.horizontalAlignment = hAlign;
        this.verticalAlignment = vAlign;
        this.horizontalGap = hGap;
        this.verticalGap = vGap;
    }
    
    /**
     * Adds a block to be managed by this instance.  This method is usually 
     * called by the {@link BlockContainer}, you shouldn't need to call it 
     * directly.
     * 
     * @param block  the block.
     * @param key  a key that controls the position of the block.
     */
    public void add(Block block, Object key) {