Using below code i'm getting list of filterable attributes

    $layer = Mage::getModel("catalog/layer");
    $attributes = $layer->getFilterableAttributes();

I can't able to set filters and get it's sub filters. example i have below 2 filterable attributes,

    Size
     28 (1)
     30 (2)
    
    Color
     Green (2)
     Red (1)

On selecting `Size 28`, my sub filter is

    Color
     Red (1)

So i'm trying to achieve below,

 - Get all available attribute filters initially (got this using `getFilterableAttributes()`)

 - Set filter programatically and get it's sub filters as like above example