In that case, anything this is not explicitly stated as illegal is legal. (Queue lecture about purpose of game manual by kmmohn). In the future, please check the search bar for the many threads about your exact question. Finally, you will be looking at the sections about what materials are legal, not the horizontal expansion section for the answer to your question. Refer to rule R7. Also command + f or control + f will be useful in finding anything related to āstringā.