@Override
    public void flush() throws IOException {
        OutputStream maybe = tee.get();
        if (maybe != null) maybe.flush();
        original.flush();
    }
Used to replace System.out and System.err, providing the ability to forward output to an additional sink.

@author jonmv
vespa-osgi-testrunner/src/main/java/com/yahoo/vespa/testrunner/TeeStream.java