let rec find p xs =
    match xs with
    | [] -> None
    | y :: ys -> if (p y) then (Some y)
                  else find p ys

let rec remove p xs =
    match xs with
    | [] -> []
    | y :: ys -> if (p y) then ys
                  else y :: (remove p ys)

let rec replace p x xs =
    match xs with
    | [] -> raise Not_found
    | y :: ys -> if (p y) then x :: ys
                   else y :: (replace p x ys)

let rec insert_before p x xs =
    match xs with
    | [] -> raise Not_found
    | y :: ys -> if (p y) then x :: y :: ys
                 else y :: (insert_before p x ys)

let rec insert_after p x xs =
    match xs with
    | [] -> raise Not_found
    | y :: ys -> if (p y) then y :: x :: ys
                 else y :: (insert_after p x ys)