code: public Name<String> nextAttachmentName(String attachmentId) {
  		NameSource result = this.attachmentNameSources.get(attachmentId);
  		if(result==null) {
  			result=new NameSource("attachment <<"+attachmentId+">>");
  		}
  		return result.nextName();
  	}
docs: Return the next available name for a given attachment. The name will be
  removed from the list of available names for the specified attachment.
  
  @param attachmentId
  the name of the attachment.
  @return the next available name for the specified attachment.