diff --git a/web/aslp.js b/web/aslp.js
index b0e3a7a..41366e4 100644
--- a/web/aslp.js
+++ b/web/aslp.js
@@ -22,6 +22,7 @@ const inputError = get('#inputerror');
 const goButton = get('#go');
 
 const debugInput = get('#debug');
+const vectorCheckbox = get('#vectors');
 const outputArea = get('#output');
 const loadingText = get('#loading');
 const downloadButton = get('#dl');
@@ -39,6 +40,25 @@ const parseIntSafe = (s, radix) => {
   return x;
 };
 
+/** Wraps the given async function, enforcing mutual exclusion across calls to this function. */
+const mutex = asyncfn => {
+  let prev = Promise.resolve();
+
+  const go = prev => async (...args) => {
+    try { await prev; }
+    finally { return asyncfn(...args); }
+  };
+
+  const wrapped = (...args) => {
+    // successive calls to the wrapped function are forced
+    // to wait for the completion of the (directly) prior call.
+    prev = go(prev)(...args);
+    return prev;
+  };
+
+  return wrapped;
+};
+
 
 const _write = (isError) => s => requestAnimationFrame(() => {
   if (!s) return;
@@ -115,7 +135,8 @@ export const shareLink = () => {
  * SEMANTICS BUTTON CLICK AND OUTPUT WRITING
  */
 
-export const submit = async () => {
+export const submit = mutex(async () => {
+  // console.log('a')
   clearOutput();
 
   try {
@@ -128,7 +149,13 @@ export const submit = async () => {
 
     copyArea.value = url.toString();
 
-    const arg = { opcode: previousOpcode, debug: parseInt(debugInput.value) };
+    const flag = bool => bool ? '+' : '-';
+
+    const arg = {
+      opcode: previousOpcode,
+      debug: parseInt(debugInput.value),
+      flags: [`${flag(vectorCheckbox.checked)}dis:vectors`], // TODO: generalise to more flags
+    };
     await worker.dis(arg);
 
   } catch (e) {
@@ -141,8 +168,9 @@ export const submit = async () => {
     dl.disabled = false;
     clearButton.disabled = false;
     shareButton.disabled = false;
+    // console.log('b')
   }
-};
+});
 
 
 /**
@@ -260,6 +288,7 @@ const init = async () => {
     if (urlData.get('bytes') != null) bytesInput.value = urlData.get('bytes');
     if (urlData.get('asm') != null) asmInput.value = urlData.get('asm');
     if (urlData.get('debug') != null) debugInput.value = urlData.get('debug');
+    if (urlData.get('vectors') != null) vectorCheckbox.checked = urlData.get('vectors') === 'on';
   } catch (exn) {
     console.error('exception during url loading:', exn);
   }
diff --git a/web/index.html b/web/index.html
index 821e2bd..ade8be7 100644
--- a/web/index.html
+++ b/web/index.html
@@ -30,7 +30,11 @@
       margin-bottom: 0.5em;
     }
 
-    input[type="radio"], label, summary {
+    fieldset {
+      width: fit-content;
+    }
+
+    input[type="radio"], label[for], summary {
       cursor: pointer;
     }
 
@@ -70,15 +74,23 @@ <h1>ASLp Web</h1>
       <!-- </p> -->
       <p>
         <details>
-          <summary>Debug options</summary>
-          <p>
-            <select name="debug" id="debug">
-              <option value="0">minimal</option>
-              <option value="1">stack trace</option>
-              <option value="2">stack trace, variables</option>
-              <option value="3">stack trace, variables, residual programs</option>
-            </select>
-          </p>
+          <summary>Additional options</summary>
+          <fieldset>
+            <p>
+              <label>Flags</label><br/>
+              <input type="checkbox" id="vectors" name="vectors" />
+              <label for="vectors">Use vectoriser</label><br/>
+            </p>
+            <p>
+            <label for="debug">Debug output</label><br/>
+              <select id="debug" name="debug" id="debug">
+                <option value="0">minimal</option>
+                <option value="1">stack trace</option>
+                <option value="2">stack trace, variables</option>
+                <option value="3">stack trace, variables, residual programs</option>
+              </select>
+            </p>
+          </fieldset>
         </details>
       </p>
 
diff --git a/web/js.ml b/web/js.ml
index 63c943e..22f86e1 100644
--- a/web/js.ml
+++ b/web/js.ml
@@ -36,7 +36,8 @@ let () =
     (object%js
       (* TODO: support stdin from javascript for repl, possibly converting asli repl to lwt *)
       method init (out : string -> unit) (err : string -> unit) = init (fun () -> "") out err
-      method printException (exn : exn) = Printexc.to_string exn;
+      method formatException (exn : exn) = Printexc.to_string exn
+      method printException (exn : exn) = output_string stderr (Printexc.to_string exn)
 
       method marshal = Lib.marshal (env ())
       method unmarshal (x : Lib.js_uint8Array) = (cachedenv := Some (Lib.unmarshal x))
@@ -45,5 +46,7 @@ let () =
       method dis (x: string) = dis x
       method setDebugLevel i = (LibASL.Dis.debug_level := i)
       method setPrettyPrint (x : bool) = (print_pp := x)
+      method setFlags (flags: Js.js_string Js.t Js.js_array Js.t) =
+        Array.iter (fun x -> LibASL.Flags.set_flag @@ Js.to_string x) (Js.to_array flags)
      end) in
   ()
diff --git a/web/worker.js b/web/worker.js
index 88d3692..bfeaaae 100644
--- a/web/worker.js
+++ b/web/worker.js
@@ -7,6 +7,7 @@ const formatOCamlExceptions = f => (...args) => {
     if (e instanceof Array) {
       // convert ocaml representation of errors into
       // javascript representation.
+      // XXX: deferring via setTimeout avoids returning this to the caller.
       setTimeout(() => { throw e; }, 0);
     } else {
       throw e;
@@ -55,8 +56,11 @@ const methods = {
     unmarshal(arraybuf);
   }),
 
-  dis: formatOCamlExceptions(({ opcode, debug }) => {
+  dis: formatOCamlExceptions((args) => {
+    const { opcode, debug, flags } = args;
     libASL_web.setDebugLevel(debug);
+    libASL_web.setFlags(flags);
+    console.log('worker dis:', args);
     libASL_web.dis(opcode);
   }),