add lazy implementation of handle_default_opt in runtime

compiler/runtime.ml

@@ -31,7 +31,7 @@ type source_position = {
   law_headings : string list;
 }
 
-type 'a eoption = ENone of unit | ESome of 'a
+type 'a eeoption = ENone of unit | ESome of 'a
 
 exception EmptyError
 
@@ -215,6 +215,25 @@ let handle_default : 'a. (unit -> 'a) array -> (unit -> bool) -> (unit -> 'a) ->
   in
   match except with Some x -> x | None -> if just () then cons () else raise EmptyError
 
+let handle_default_opt: 'a. 'a eoption array -> (unit -> bool eoption) -> (unit -> 'a eoption) -> 'a eoption = 
+  fun exceptions just cons ->
+    let except = 
+      Array.fold_left
+      (fun acc except ->
+        match acc, except with
+        | ENone, _ -> except
+        | ESome _, ENone -> acc
+        | ESome _, ESome _ -> raise ConflictError)
+      None exceptions
+    in
+    match except with
+    | ESome _ -> except
+    | ENone -> begin
+      match just () with
+      | ESome b -> if b then cons () else ENone
+      | ENone -> ENone
+    end
+
 let no_input : unit -> 'a = fun _ -> raise EmptyError
 
 let ( *$ ) (i1 : money) (i2 : decimal) : money =

compiler/runtime.mli

@@ -175,6 +175,9 @@ val handle_default : (unit -> 'a) array -> (unit -> bool) -> (unit -> 'a) -> 'a
 (** @raise EmptyError
     @raise ConflictError *)
 
+val handle_default_opt: 'a option array -> (unit -> bool option) -> (unit -> 'a option) -> 'a option
+(** @raise ConflictError *)
+
 val no_input : unit -> 'a
 
 (**{1 Operators} *)