Find all invoice dates corresponding to customers with first name Astrid and last name Gruber.
return customers ;return first names of  #1 ;return #1 where #2 is  Astrid ;return last names of  #1 ;return #1 where #2 is  Gruber ;return #1 of  both #3 and  #5 ;return invoice dates corresponding to #6
['select', 'project', 'comparative', 'project', 'comparative', 'intersection', 'project']