dyn flies(X). dyn bird(X). default birdsfly(X): flies(X) <- bird(X). constraint not birdsfly(X) <- not flies(X). default emusdontfly(X): not flies(X) <- emu(X) . constraint not emusdontfly(X) <- flies(X). constraint not birdsfly(X) <- emu(X). fact bird(X) <- emu(X). fact bird(X) <- robin(X). fact bird(tweety). fact emu(polly). fact robin(cohen).