diff --git a/packages/compiler/src/compiler/codegen.ts b/packages/compiler/src/compiler/codegen.ts
index 7d45de7..847368c 100644
--- a/packages/compiler/src/compiler/codegen.ts
+++ b/packages/compiler/src/compiler/codegen.ts
@@ -148,25 +148,29 @@ export function expand(tree: Items, ctx: ExpansionContext): Items {
             spawn.linkedToken = getRange(s.body);
         }
 
-        let body = (spawn == null)
+        let body = (spawn === null)
             ? walk(s.body)
             : expandSpawn(spawn, t, t`__SYNDICATE__.Turn.activeFacet.preventInertCheck();`);
 
         const sa = compilePattern(s.pattern);
-        return t`assertDataflow(() => ({
-            target: currentSyndicateTarget,
-            assertion: __SYNDICATE__.fromObserve(__SYNDICATE__.Observe({
-                pattern: __SYNDICATE__.QuasiValue.finish(${sa.skeleton}),
-                observer: __SYNDICATE__.Turn.ref(__SYNDICATE__.assertionFacetObserver(
-                    (${ctx.argDecl(t, '__vs', '__SYNDICATE__.AnyValue')}) => {
-                        if (Array.isArray(__vs)) {
+        const assertion = t`__SYNDICATE__.fromObserve(__SYNDICATE__.Observe({
+            pattern: __SYNDICATE__.QuasiValue.finish(${sa.skeleton}),
+            observer: __SYNDICATE__.Turn.ref(__SYNDICATE__.assertionFacetObserver(
+                (${ctx.argDecl(t, '__vs', '__SYNDICATE__.AnyValue')}) => {
+                    if (Array.isArray(__vs)) {
 ${joinItems(sa.captureBinders.map(binderTypeGuard(ctx, t)), '\n')}
-                            ${body}
-                        }
+                        ${body}
                     }
-                ))
-            })),
-        }));`;
+                }
+            ))
+        }))`;
+        if (s.test === void 0) {
+            return t`assertDataflow(() => ({ target: currentSyndicateTarget, assertion: ${assertion} }));`;
+        } else {
+            return t`assertDataflow(() => (${walk(s.test)})
+                   ? ({ target: currentSyndicateTarget, assertion: ${assertion} })
+                   : ({ target: void 0, assertion: void 0 }));`;
+        }
     });
 
     function expandSpawn(spawn: SpawnStatement, t: TemplateFunction, inject: Items = []): Items {
@@ -197,7 +201,7 @@ ${joinItems(sa.captureBinders.map(binderTypeGuard(ctx, t)), '\n')}
 
     xf(ctx.parser.assertionEndpointStatement, (s, t) => {
         if (s.isDynamic) {
-            if (s.test == void 0) {
+            if (s.test === void 0) {
                 return t`assertDataflow(() => ({ target: currentSyndicateTarget, assertion: ${walk(s.template)} }));`;
             } else {
                 return t`assertDataflow(() => (${walk(s.test)})
@@ -205,7 +209,7 @@ ${joinItems(sa.captureBinders.map(binderTypeGuard(ctx, t)), '\n')}
                        : ({ target: void 0, assertion: void 0 }));`;
             }
         } else {
-            if (s.test == void 0) {
+            if (s.test === void 0) {
                 return t`assert(currentSyndicateTarget, ${walk(s.template)});`;
             } else {
                 return t`replace(currentSyndicateTarget, void 0, (${walk(s.test)}) ? (${walk(s.template)}) : void 0);`;
diff --git a/packages/compiler/src/compiler/grammar.ts b/packages/compiler/src/compiler/grammar.ts
index d07632a..adb379d 100644
--- a/packages/compiler/src/compiler/grammar.ts
+++ b/packages/compiler/src/compiler/grammar.ts
@@ -85,6 +85,7 @@ export interface MessageSendStatement extends TurnAction {
 
 export interface DuringStatement extends FacetSetupAction {
     pattern: ValuePattern;
+    test?: Expr,
 }
 
 export interface ReactStatement extends FacetSetupAction {
@@ -314,7 +315,9 @@ export class SyndicateParser {
         this.turnAction(o => {
             o.body = [];
             return seq(atom('during'),
-                       bind(o, 'pattern', this.valuePattern(1, atom('=>'))),
+                       bind(o, 'pattern',
+                            this.valuePattern(1, atom('=>'), seq(atom('when'), group('(', discard)))),
+                       option(seq(atom('when'), group('(', bind(o, 'test', this.expr())))),
                        seq(atom('=>'), this.statement(o.body)));
         });