public DocumentProcessor.Progress processOnce(ComponentSpecification chain, com.yahoo.docproc.Processing processing) {
        DocprocExecutor executor = getExecutor(chain);
        return executor.process(processing);
    }
Processes the given Processing through the specified chain. Note that if one
{@link com.yahoo.docproc.DocumentProcessor DocumentProcessor} in the
chain returns a {@link com.yahoo.docproc.DocumentProcessor.LaterProgress DocumentProcessor.LaterProgress},
it will be returned by this method. This method will hence return whenever a document processor returns any
{@link com.yahoo.docproc.DocumentProcessor.Progress DocumentProcessor.Progress}, or
throws an exception.

@param chain the specification of the chain to execute
@param processing the Processing to process
@return any Progress
@throws RuntimeException if one of the document processors in the chain throws
application/src/main/java/com/yahoo/application/container/DocumentProcessing.java