Question 142
There is an infinite sequence of letters $a$ and $b$. In this sequence, one can perform the substitution $abb \rightarrow baa$. It is known that, regardless of the order of these substitutions, one can only make a finite number of them. Prove that, in that case, substitutions $aabb \rightarrow bbaa$ can also be made only a finite number of times.
(Arsenii Nikolaiev)
(Arsenii Nikolaiev)