Exercise 1.11: A function 
  f
 is defined by
the rule that 
  
    f
    (
    n
    )
    =
    n
  
 if 
  
    n
    <
    3
  
 and 
  
    f
    (
    n
    )
  
  =
  
    f
    (
    n
    −
    1
    )
  
  +
  
    2
    f
    (
    n
    −
    2
    )
  
  +
  
    3
    f
    (
    n
    −
    3
    )
  
 if 
  
    n
    ≥
    3
  
.  
Write a procedure that computes 
  f
 by means of a recursive process.  Write a procedure that
computes 
  f
 by means of an iterative process.