Exercise 3.69: Write a procedure triples
that takes three infinite streams, 
  S
, 
  T
, and 
  U
, and produces the
stream of triples 
  
    (
    
      S
      i
    
    ,
    
      T
      j
    
    ,
    
      U
      k
    
    )
  
 such that 
  
    i
    ≤
    j
    ≤
    k
  
.  
Use triples to generate the stream of all Pythagorean
triples of positive integers, i.e., the triples 
  
    (
    i
    ,
    j
    ,
    k
    )
  
 such that

  
    i
    ≤
    j
  
 and 
  
    
      i
      2
    
    +
    
      j
      2
    
    =
    
      k
      2
    
  
.