Recursion: Fibonacci String

A Fibonacci string is a precedence of the Fibonacci series. It works with any two characters of the English alphabet (as opposed to the numbers 0 and 1 in the Fibonacci series) as the initial items and concatenates them together as it progresses similarly to that of the Fibonacci series.

Examples

fib_str(3, ["j", "h"]) ➞ "j, h, hj"

fib_str(5, ["e", "a"]) ➞ "e, a, ae, aea, aeaae"

fib_str(6, ["n", "k"]) ➞ "n, k, kn, knk, knkkn, knkknknk"

Notes

• All values for n will be at least 2.
• It is expected from the challenge-takers to come up with a solution using the concept of recursion or the so-called recursive approach.
• You can read more topics about recursion if you aren't familiar with it yet or hasn't fully understood the concept behind it before taking up this challenge or unless otherwise.
• An iterative version of this challenge can be found via this link.