What are the invoice dates for customers with the first name Astrid and the last name Gruber?
return customers ;return first names of  #1 ;return #1 where #2 is  Astrid ;return last names of  customers ;return #1 where #4 is  Gruber ;return #1 of  both #3 and  #5 ;return invoices of  #6 ;return dates of  #7
['select', 'project', 'comparative', 'select', 'comparative', 'intersection', 'project', 'project']