Write a function called filter-seq that takes a sequence s (a string) and an alphabet a (a string), and generates a result sequence (a string) by including only the characters of s that occur in the alphabet a. The unit test below includes test cases that your solution for this exercise must pass.

test:

#+begin_src lisp (deftest test-filter-seq () (check (equal (filter-seq "sdafgacs" "abcf") "afac") (equal (filter-seq "" "abcf") "") (equal (filter-seq "sdafgacs" "") "") (equal (filter-seq "sdafgacs" "abcfs") "safacs"))) #+end_src lisp