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).
