Exercise 2.40: Define a procedure
unique-pairs that, given an integer 
  n
, generates the sequence of
pairs 
  
    (
    i
    ,
    j
    )
  
 with 
  
    1
    ≤
    j
  
  <
  
    i
    ≤
    n
  
.  Use unique-pairs
to simplify the definition of prime-sum-pairs given above.