protected static int calculateEmptyHashCode() {
		int hash = MurmurHash.initialize(INITIAL_HASH);
		hash = MurmurHash.finish(hash, 0);
		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