3. Consider each of the following pseudocode fragment S with the stated Pre-assertion "Pre". For code fragment S1, do items (a), (b), and (c); for code fragment S2, do items (a) and (b). (a) What does the code fragment compute and provide the corresponding post-assertion "Post"? (b) Follow the approach described in our handout (lect21_01_20-2.pdf) and lecture example on Program Correctness and Documentation: provide a loop invariant for the while-loop and prove its correctness (by placing appropriate assertions between all body/constituent statements). (c) Prove // Pre S // Post for the stated Pre-assertion with your provided Post-assertion in item (a). Code fragment S1 : // Pre: integer N is defined and >=0, and integer n=N. f:=1 while not (n=0) loop f:=fn n:=n1 end loop; // Post: Code fragment S2 : // Pre: integers a and b are defined, and a>=0 and b>0. x:=a; y:=0 while b