ocaml language code

Write a function countc : string -> char -> int such that countc sc counts the number of times character c appears in the string s. So for example countc "acab" 'a' should evaluate to 2; countc "birdofprey" 'z' should evaluate to 0; and countc "zzzzz" 'z' should evaluate to 5. Hint: you will probably find it useful to define a helper function that counts the number of times c appears between either the beginning of the string and a given index or the end of the string and a given index