Derive scalar changes and composition laws

src/util.ml

@@ -34,8 +34,16 @@ let rec drop count items =
 
 let contains value items = List.exists (fun item -> item = value) items
 
+let index_of_char text character =
+  let rec search index =
+    if index >= String.length text then None
+    else if text.[index] = character then Some index
+    else search (index + 1)
+  in
+  search 0
+
 let string_before_char text character =
-  match String.index_opt text character with
+  match index_of_char text character with
   | Some index -> String.sub text 0 index
   | None -> text