Can't find a clear example of disallowing parallel edges

From: Arthur (arthurc_at_[hidden])
Date: 2003-06-05 09:44:34

I'm trying to use this graph class without detailed reading.  I can't 
find a decent, clear example of doing this.  I have a simple graph:

typedef adjacency_list<list, listS, bidirectionalS> Graph;

What do I do to prevent parallel edges to be inserted?  

Thanks for any help offered,
Arthur