public void setTee(OutputStream tee) {
        if ( ! this.tee.compareAndSet(null, tee)) throw new IllegalStateException("tee already set");
    }
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