A list of the top 10 countries by average invoice size. List country name and average invoice size.
return countries ;return invoice sizes of #1 ;return the  average of #2 for each #1 ;return the  top 10 highest of #3 ;return #1 where #3 is equal to any of #4 ;return names of #5 ;return #6 ,  #4
['select', 'project', 'group', 'aggregate', 'comparative', 'project', 'union']