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Edge_iterator
        
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    e.edge_node ()
        
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    returns an iterator to the edge node in the hierarchy tree which
       holds the curve from 
       which the halfedge was generated. This enables
       traversal over the hierarchy tree that e belongs to.
       For example, if eit is the returned iterator, 
       eit->curve_node() will return the root of the hierarchy tree and
       ++eit
       will return the following edge in the hierarchy tree (unless eit
       is the rightmost edge in the hierarchy tree, in which case ++eit
       will return the past-the-end value.)
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