feat: support for model-requested horizons

exm/zelus/ballz/ballz.zls

@@ -9,8 +9,7 @@ let hybrid ball (y0, y'0) = (y, y', z) where
 
 let hybrid main () =
   let der t = 1.0 init 0.0 in
-  let rec der p = 1.0 init -0.01 reset s -> -0.01
-      and s = up(p) in 
+  let s = period(0.01) in
   let (y, y', z) = ball (y0, y'0) in
   present z | s -> (
     print_float t;

exm/zelus/ballz/dune

@@ -1,6 +1,6 @@
 (rule
- (targets ballz.ml ballz.zci)
+ (targets ballz.ml ballz.zci ballz_main.ml)
  (deps
   (:zl ballz.zls))
  (action
-  (run zeluc %{zl})))
+  (run zeluc -s main -o ballz_main %{zl})))