create index in SAS using do loop

Question

Say I have a set of data in this format:

ID  Product account open date  
1   A   20100101  
1   B   20100103  
2   C   20100104  
2   A   20100205  
2   D   20100605  
3   A   20100101  

And I want to create a column to capture the sequence of the products opened so the table will look like this:

ID  First   Second  third  
1   A   B   
2   C   A   D  
3   A       

I know I need to create an index for each ID so I can transpose the data afterwards:

ID  Product account open date   sequence  
1   A   20100101    1  
1   B   20100103    2  
2   C   20100104    1  
2   A   20100205    2  
2   D   20100605    3  
3   A   20100101    1 

From my limited knowledge in do loop, I think I need to write something like this:
if first.ID and not last.ID then n=1 do while ID not last n+1

Something like that. Can anyone help me with the exact syntax? I have tried googling for similar codes and haven't had much luck.

Thanks!