4. A game of Yasuhara tag. The formal language Y has three symbols a, b, and c. The formulas are words in these symbols. The only axiom is abccba. There are three rules of inference: a-rule: If a formula begins with a, add bab to the right and then erase the first three symbols. b-rule: If a formula begins with b, add abba to the right and then erase the first three symbols. c-rule: If a formula begins with c, add ca to the right and then erase the first three symbols. List all theorems of Y. Is the language Y decidable? 4. A game of Yasuhara tag. The formal language Y has three symbols a, b, and c. The formulas are words in these symbols. The only axiom is abccba. There are three rules of inference: a-rule: If a formula begins with a, add bab to the right and then erase the first three symbols. b-rule: If a formula begins with b, add abba to the right and then erase the first three symbols. c-rule: If a formula begins with c, add ca to the right and then erase the first three symbols. List all theorems of Y. Is the language Y decidable