Find the first name of students living in city PHL whose age is between 20 and 25.
return students ;return cities of #1 ;return #1 where #2 is PHL ;return ages of #3 ;return #3 where #4 is at least 20 ;return #3 where #4 is at most 25 ;return #3 in  both #5 and  #6 ;return first names of #7
['select', 'project', 'comparative', 'project', 'comparative', 'comparative', 'intersection', 'project']