let g = 9.81
let y0 = 0.0
let y'0 = 10.0

let hybrid ball () = (y, y', z) where
  rec der y  = y'   init y0
  and der y' = -. g init y'0 reset z -> -0.8 *. (last y')
  and z = up(-. y)

let hybrid main () =
  let der t = 1.0 init 0.0 in
  let s = period(0.01) in
  let (y, y', z) = ball () in
  present z | s -> (
    print_float t;
    print_string "\t";
    print_float y;
(*    print_string "\t";
    print_float y'; *)
    print_newline ()
  ); ()