How many balls are of the same size and made of the same material?
return balls ;return sizes of #1 ;return materials that  #1 are made of ;return #1 where  #2 are the  same  ;return #1 where  #3 are the  same  ;return #1 in  both  #4 and #5 ;return number of #6
['select', 'project', 'project', 'comparative', 'comparative', 'intersection', 'aggregate']