public static Reply merge(List<Reply> replies) {
        return merge(replies, new HashSet<>(0)).second;
    }
This method implements the common way to merge document replies for whatever routing policy. In case of an error
in any of the replies, it will prepare an EmptyReply() and add all errors to it. If there are no errors, this
method will use the first reply in the list and transfer whatever feed answers might exist in the replies to it.


@param replies the replies to merge
@return the merged Reply
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/DocumentProtocol.java