protected static int calculateHashCode(PredictionContext parent, int returnState) {
		int hash = MurmurHash.initialize(INITIAL_HASH);
		hash = MurmurHash.update(hash, parent);
		hash = MurmurHash.update(hash, returnState);
		hash = MurmurHash.finish(hash, 2);
		return hash;
	}
This means only the {@link EmptyPredictionContext#Instance} (wildcard? not sure) context is in set.
runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContext.java