If there are six dog pulling two people on the sled, over snow.
return sled ;return people on #1 ;return number of  #2 ;return if  #3 is equal to  two ;return dog ;return #5 pulling #2 over snow ;return number of  #6 ;return if  #7 is equal to  six ;return if  both  #4 and #8
['select', 'project', 'aggregate', 'boolean', 'select', 'filter', 'aggregate', 'boolean', 'boolean']