let hd (x::xs) = x
let tl (x::xs) = xs

let rec delete x ls1 = match ls1 with
| [] -> []
| x1::tl -> if x1 = x then delete x tl else x1::(delete x tl)
| x1::[] -> if x1 = x then delete x [] else [x1]

delete 7 [7;8;23;7;9;2]