What are the ids of the students who are under 20 years old and are involved in at least one activity.
return students ;return #1 under 20 years old ;return activities of  #1 ;return number of  #3 for each  #1 ;return #1 where #4 is  at least one ;return #1 of  both #2 and  #5 ;return ids of  #6
['select', 'filter', 'project', 'group', 'comparative', 'intersection', 'project']