in slf4j-to-log4j-api/src/main/java/org/apache/logging/slf4j/Log4jMarker.java [49:55]
public void add(final Marker marker) {
if (marker == null) {
throw new IllegalArgumentException();
}
final Marker m = factory.getMarker(marker.getName());
this.marker.addParents(((Log4jMarker) m).getLog4jMarker());
}