dt filter out uvar in input mode with rigid arg

[FissoreD]

I have tested this change on all the project thanks to this change, where I put DTree
in all predicate with high indexing depth.

To perform the same test apply the following diff.

diff --git a/src/compiler/compiler.ml b/src/compiler/compiler.ml
index 983da7c7..e134e47e 100644
--- a/src/compiler/compiler.ml
+++ b/src/compiler/compiler.ml
@@ -512,13 +512,13 @@ end = struct (* {{{ *)
       | Index(i,index_type) :: rest ->
          let it =
            match index_type with
-           | None -> None
-           | Some "Map" -> Some Map
-           | Some "Hash" -> Some HashMap
+           | None -> Some DiscriminationTree
+           | Some "Map" -> Some DiscriminationTree
+           | Some "Hash" -> Some DiscriminationTree
            | Some "DTree" -> Some DiscriminationTree
            | Some s -> error ~loc ("unknown indexing directive " ^ s ^ ". Valid ones are: Map, Hash, DTree.") in
          begin match r with
-           | None -> aux_tatt (Some (Structured.Index(i,it))) f rest
+           | None -> aux_tatt (Some (Structured.Index(List.init 100 (fun _ -> 100),it))) f rest
            | Some (Structured.Index _) -> duplicate_err "index"
            | Some _ -> error ~loc "external predicates cannot be indexed"
          end
diff --git a/src/runtime/runtime.ml b/src/runtime/runtime.ml
index 471bed38..6f14124c 100644
--- a/src/runtime/runtime.ml
+++ b/src/runtime/runtime.ml
@@ -2629,7 +2629,7 @@ let arg_to_trie_path ~safe ~depth ~is_goal args arg_depths args_depths_ar mode m
       make_sub_path arg_hd arg_tl arg_depth_hd arg_depth_tl Output []
     | arg_hd :: arg_tl, arg_depth_hd :: arg_depth_tl, mode_hd :: mode_tl ->
       make_sub_path arg_hd arg_tl arg_depth_hd arg_depth_tl (get_arg_mode mode_hd) mode_tl 
-    | _, _ :: _,_ -> anomaly "Invalid Index length" in
+    | _, _ :: _,_ -> () in
   begin
     if args == [] then emit_mode is_goal mkOutputMode
     else aux ~safe ~depth is_goal args (if is_goal then Array.to_list args_depths_ar else arg_depths) mode