let set_end lab coord =
  let (i,j) = coord in
  let (signal, set) = lab.(i).(j) in
  let v = S.value signal in
  set {v with finish = true }

let empty_lab n m finish =
  let lab =
    Array.init n
           (fun _ ->
        Array.init m
               (fun _ ->
                let tmp : 'a React.signal * ( case -> unit) =
                  React.S.create
                {visited=false;
                 beginning=false;
                 finish=false;
                 borders=Array.make 4 false;} in
                tmp ))
  in
  set_end lab finish; (* <==== ERROR HERE *)
  lab


Error: This expression has type
         case React.signal * (?step:React.step -> case -> unit)
       but an expression was expected of type
         case React.signal * (case -> unit)
       Type ?step:React.step -> case -> unit is not compatible with type
         case -> unit