public Progress process(Processing processing) {
            Document document = ((DocumentPut)processing.getDocumentOperations().get(0)).getDocument();
            document.removeFieldValue("name");
            return Progress.DONE;
        }
To make it less likely to break schema mapping, we enforce that ProxyDocument does wrap every public
non-static, non-final method on Document and StructuredFieldValue
docproc/src/test/java/com/yahoo/docproc/proxy/SchemaMappingAndAccessesTest.java