What material is the smallestt sphere, metal or rubber?
return sphere ;return size of #1 ;return #1 where #2 is  the  smallestt ;return the  material of #3 ;return if  #4 is  metal ;return if  #4 is  rubber ;return #5 or  #6
['select', 'project', 'comparative', 'project', 'boolean', 'boolean', 'union']