@Override
    public String put(String key, String value) {
        if (value == null)    throw new IllegalArgumentException("Null value not allowed as an environment variable: " + key);
        return super.put(key, value);
    }
@Test
    public void overrideOrderCalculatorSelfReference() {
        EnvVars env = new EnvVars();
        EnvVars overrides = new EnvVars();
        overrides.put("PATH", "some;${PATH}");

        OverrideOrderCalculator calc = new OverrideOrderCalculator(env, overrides);
        List<String> order = calc.getOrderedVariableNames();
        assertEquals(List.of("PATH"), order);
    }