Implement the sequence class from Section 4.5. You may wish to provide some additional useful methods, such as: 

(1) A method to add a new element at the front of the sequence; 

(2) A method to remove the element at the front of the sequence; 

(3) A method to add a new element at the end of the sequence; 

(4) A method that makes the last element of the sequence become the current element; 

(5) A method that returns the i^th element of the sequence (starting with the 0^th at the front);

(6) A method that makes the i^th element become the current element.