public Value get(Key1 k1, Key2 k2) {
		Map<Key2, Value> data2 = data.get(k1);
		if ( data2==null ) return null;
		return data2.get(k2);
	}
Sometimes we need to map a key to a value but key is two pieces of data.
 This nested hash table saves creating a single key each time we access
 map; avoids mem creation.
runtime/Java/src/org/antlr/v4/runtime/misc/DoubleKeyMap.java