urce is a "multipart/signed" data source may
 return an appropriate subclass of Multipart. <p>

 Some messaging systems provide different subtypes of Multiparts. For
 example, MIME specifies a set of subtypes that include "alternative", 
 "mixed", "related", "parallel", "signed", etc. <p>

 Multipart is an abstract class.  Subclasses provide actual implementations.</div>
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