[Tutor] What am I missing on the index?

[DavidCraig@PIA.CA.GOV]

Sorry to bother you all again.  I have been trying this one all weekend.
What am I doing wrong.  This is an exercise from How to Think Like a
Computer Scientist (Python):

The exercise is to write a function that takes a string as an argument that
and outputs the letters backwards, one per line.

## stringloop function prints a string backwards

def stringback(string):
    string = "David"
    index = 0
    while index < len(string):
        letter = string[index]
        print letter
        index = index - 1

And my output back is:

Python 2.1 (#15, Apr 16 2001, 18:25:49) [MSC 32 bit (Intel)] on win32
Type "copyright", "credits" or "license" for more information.
IDLE 0.8 -- press F1 for help
>>>
stringback(string)
D
d
i
v
a
D
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in ?
    stringback(string)
  File "C:/Python21/Practice/stringback.py", line 7, in stringback
    letter = string[index]
IndexError: string index out of range


Any help would be appreciated.  I just can't seem to see it.

Thanks , Dave
D. H. Craig, CSM