let rec find p xs =
match xs with
| [] -> None
| x :: xs' -> if (p x) then (Some x)
else find p xs'
let rec remove p xs =
match xs with
| [] -> []
| x :: xs' -> if (p x) then xs'
else x :: (remove p xs)
let rec replace p x xs=
match xs with
| [] -> []
| x' :: xs' -> if (p x') then x :: xs'
else x' :: (replace p x xs')