elk-reasoner/src/main/java/org/semanticweb/elk/reasoner/indexing/classes/CachedIndexedComplexClassExpressionImpl.java
public final void checkOccurrenceNumbers() {
		if (LOGGER_.isTraceEnabled())
			LOGGER_.trace(toString() + " occurences: "
					+ printOccurrenceNumbers());
		if (positiveOccurrenceNo < 0 || negativeOccurrenceNo < 0)
			throw new ElkUnexpectedIndexingException(toString()
					+ " has a negative occurrence: " + printOccurrenceNumbers());
	}
public final void checkOccurrenceNumbers() {
		if (LOGGER_.isTraceEnabled())
			LOGGER_.trace(toString() + " occurences: "
					+ printOccurrenceNumbers());
		if (positiveOccurrenceNo < 0 || negativeOccurrenceNo < 0)
			throw new ElkUnexpectedIndexingException(toString()
					+ " has a negative occurrence: " + printOccurrenceNumbers());
	}
verifies that occurrence numbers are not negative