sa : swap a - swap the first 2 elements at the top of stack a. Do nothing if there is only one or no elements). sb : swap b - swap the first 2 elements at the top of stack b. Do nothing if there is only one or no elements). ss : sa and sb at the same time. pa : push a- take the first element at the top of b and put it at the top of a. Do nothing if b is empty. pb : push b- take the first element at the top of a and put it at the top of b. Do nothing if a is empty. ra : rotate a- shift up all elements of stack a by 1. The first element becomes the last one. rb : rotate b- shift up all elements of stack b by 1. The first element becomes the last one. rr : ra and rb at the same time. rra : reverse rotate a- shift down all elements of stackaby 1. The last element becomes the first one. rrb : reverse rotate b- shift down all elements of stackbby 1. The last element becomes the first one. rrr : rra and rrb at the same time