let is_subseq s t =
  let i = ref 0 in
  let n = String.length s in
  let m = String.length t in
  let j = ref 0 in
  while !i < n && !j < m do
    if s.[!i] = t.[!j] then i := !i + 1;
    j := !j + 1
  done;
  !i = n

;;

(if is_subseq "abc" "ahbgdc" then 1 else 0) +
(if is_subseq "axc" "ahbgdc" then 1 else 0) +
(if is_subseq "" "anything" then 1 else 0) +
(if is_subseq "abc" "abc" then 1 else 0) +
(if is_subseq "abcd" "abc" then 1 else 0)