Fibonacci String

For a string $S$ let the unique set of characters that occur in it one or more times be $C$. Consider a permutation of the elements of $C$ as $(c_1, c_2, c_3 ... )$. Let $f(c)$ be the number of times $c$ occurs in $S$. If any such permutation of the elements of $C$ satisfies $f(c_i) = f(c_{i-1}) + f(c_{i-2})$ for all $i \ge 3$, the string is said to be a **dynamic string**. Mr Bancroft is given