def d(b, a, bnds):
    """
    Calculate difference between two images
    
    Parameters:
        b (ee.Image): 'before' image
        a (ee.Image): 'after' image
        bnds (ee.List<str>): band names included in calculation
        
    Returns:
        ee.Image: difference image with original bands
    """    
    return b.select(bnds).subtract(a.select(bnds))