human
Solve the following python programming problem given the following code:
def remove_unreferenced_images(asset, default = True) : 
    def _unreference_image(imageName) : 
        if (type (imageName) is StringType) : 
            if (imageName in unreferenced_images) : 
                unreferenced_images.remove (imageName) 
images = asset.asset ['images'] 
    unreferenced_images = set (asset.asset ['images'].iterkeys ()) 
    materials = asset.asset ['materials'] 
    effects = asset.asset ['effects'] 
    for (v, _) in effects.iteritems () : 
        effect = asset.retrieve_effect (v) 
        parameters = effect.get ('parameters', None) 
        if parameters : 
            for value in parameters.itervalues () : 
                _unreference_image (value) 
for (v, _) in materials.iteritems () : 
        material = asset.retrieve_material (v, images) 
        parameters = material.get ('parameters', None) 
        if parameters : 
            for value in parameters.itervalues () : 
                _unreference_image (value) 
for i in unreferenced_images : 
        del images [i] 

    
    

    Fix the buggy line: material = asset.retrieve_material (v, images)
assistant
#Here is Python program to fix the buggy line: material = asset.

material = asset.retrieve_material (v, default)