  This way of doing things makes any case in which a fluent changes truth value a prima facie anomaly. But it follows from Reiter’s account of extensions that such defaults are overridden when they conflict with the monotonic theory of situation dynamics. So if, for instance, there is a monotonic causal axiom for the action blacken ensuring that blackening a block will make it black in the resulting situation, then the appropriate instance of IR will be inefficacious, and there will be no extension in which a white block remains white when it is blackened.