@Override
    public DocumentOperation getWrappedDocumentOperation() {
        DocumentOperation innermostDocOp = docU;
        while (innermostDocOp instanceof DocumentOperationWrapper) {
            innermostDocOp = ((DocumentOperationWrapper) innermostDocOp).getWrappedDocumentOperation();
        }
        return innermostDocOp;
    }
The field name map for schema mapping. The key is the field name that the docproc uses. 
The value is the actual name of the field in the document.
docproc/src/main/java/com/yahoo/docproc/proxy/ProxyDocumentUpdate.java