Find the id of instructors who taught a class in Fall 2009 but not in Spring 2010.
return classes ;return #1 in  Fall 2009 ;return #1 in  Spring 2010 ;return instructors of #2 ;return instructors of both #2 and  #3 ;return #4 besides  #5 ;return ids of #6
['select', 'filter', 'filter', 'project', 'intersection', 'discard', 'project']