While implementing the LocalId and WebServiceAddresses interfaces for the e-d-s backend, I've noticed that when you try to link e-d-s contacts by (say) web-service-addresses, you'll implicitly have a link via local ids too. 

Is this side-effect something desirable? 

I think that link_personas() should have a link_by parameter that could be an enum defined (something) like this:

public enum Folks.LinkBy {
   FIELD_ANY,
   FIELD_ALL,
   FIELD_IM_ADDRESSES,
   FIELD_WEB_SERVICE_ADDRESSES,
   FIELD_LOCAL_IDS
}

and the default value (by omission) would be folks.LinkBy.FIELD_ALL (which is the current behaviour).

Even though in practice users might end up going with FIELD_ALL most of the times, at least for testing purpose its nice to be able to say what you want instead of having Folks creating as many links as it can. 

Thoughts?