What is the last name of every student who is either female or living in a city with the code BAL or male and under 20?
return students ;return #1 who  are  female ;return cities of  #1 ;return codes of  #3 ;return #1 where #4 is  BAL ;return #1 who  are  male ;return #6 who  are  under 20 ;return #2 ,  #5 ,   #7
['select', 'filter', 'project', 'project', 'comparative', 'filter', 'filter', 'union']