private DocumentUpdate configDocUpd(DocumentProcessor proc, DocumentUpdate docU) {
        if (proc.getFieldMap().isEmpty()) return docU;
        return new ProxyDocumentUpdate(docU, proc.getDocMap(docU.getDocumentType().getName()));
    }
The DocumentUpdate object a processor should work on. The one in args, or schema mapped.

@return a DocumentUpdate
docproc/src/main/java/com/yahoo/docproc/Call.java