6. On a far away galaxy, Aliens developed a (weird) limited language using only the words "good", "mood" and " food". They make new words by concatenating (juxtaposing) these words in any order. For example foodgoodfood" is a valid word in their language. Any word produced like this is a correct word in their language. Prove using strong induction that any word in this language has an even number of o's. (Hint: Use strong induction on the number of sub-words in the word)