public FactMapping addFactMapping(int index, FactMapping toClone) {
        FactMapping toReturn = toClone.cloneFactMapping();
        factMappings.add(index, toReturn);
        return toReturn;
    }
@Test
    public void addFactMapping_byFactIdentifierAndExpressionIdentifier_fail() {
        modelDescriptor.addFactMapping(factIdentifier, expressionIdentifier);

        assertThatIllegalArgumentException().isThrownBy(() -> modelDescriptor.addFactMapping(factIdentifier, expressionIdentifier));
    }