Defining the sequence of parties associated to a product in a given role

[Jacopo Cappellato]

I have to implement the following simple story: a product may have several 
parties associated to it with the same role; there should be a way to define 
the sequence of parties associated to the product in that role.

I would like to implement this by adding the sequenceNum field to the 
ProductRole entity (using the same pattern already used in WebSiteRole, 
ProductCatalogRole, ProductStoreRole).

Jacopo