@Test(expected = IllegalStateException.class)
public void testImmutablesCanOnlySetOnceFromSet() {

  VariableContext variables = new VariableContext();
  ImmutableSet<String> immutables =
      ImmutableSet.of("%user", "%primary_group", "%secondary_group");
  variables.setImmutables(immutables);
  variables.setImmutables(immutables);
}