diff --git a/src/common/ecs/compiler.ts b/src/common/ecs/compiler.ts
index 65394149..7e20c26f 100644
--- a/src/common/ecs/compiler.ts
+++ b/src/common/ecs/compiler.ts
@@ -294,7 +294,7 @@ export class ECSCompiler extends Tokenizer {
         scope.filePath = this.path;
         this.currentScope = scope;
         let cmd;
-        while ((cmd = this.expectTokens(['using', 'entity', 'scope', 'comment', 'end']).str) != 'end') {
+        while ((cmd = this.expectTokens(['end', 'using', 'entity', 'scope', 'comment', 'on']).str) != 'end') {
             if (cmd == 'using') {
                 this.parseScopeUsing();
             }
@@ -307,6 +307,9 @@ export class ECSCompiler extends Tokenizer {
             if (cmd == 'comment') {
                 this.expectTokenTypes([ECSTokenType.CodeFragment]);
             }
+            if (cmd == 'on') {
+                this.currentScope.addAction(this.annotate(() => this.parseAction()));
+            }
         }
         this.currentScope = scope.parent || null;
         return scope;
@@ -315,7 +318,7 @@ export class ECSCompiler extends Tokenizer {
     parseScopeUsing() {
         let syslist = this.parseList(this.parseSystemRef, ',');
         for (let sys of syslist) {
-            this.currentScope?.systems.push(sys);
+            this.currentScope?.addUsingSystem(sys);
         }
     }
 
diff --git a/src/common/ecs/ecs.ts b/src/common/ecs/ecs.ts
index 1fb707dd..05e49631 100644
--- a/src/common/ecs/ecs.ts
+++ b/src/common/ecs/ecs.ts
@@ -647,6 +647,14 @@ class ActionEval {
         let bitofs = parseInt(args[1] || '0');
         return this.generateCodeForField(fieldName, bitofs, canwrite);
     }
+    __index(args: string[]) {
+        let ident = args[0]; // TODO?
+        if (this.entities.length == 1) {
+            return this.dialect.absolute(ident);
+        } else {
+            return this.dialect.indexed_x(ident, 0); //TODO?
+        }
+    }
     __use(args: string[]) {
         return this.scope.includeResource(args[0]);
     }
@@ -685,7 +693,7 @@ class ActionEval {
         }
         return code;
     }
-    generateCodeForField(fieldName: string, bitofs: number, canwrite: boolean): string {
+    generateCodeForField(fieldName: string, bitofs: number, canWrite: boolean): string {
         const action = this.action;
         const qr = this.jr || this.qr; // TODO: why not both!
 
@@ -707,12 +715,14 @@ class ActionEval {
         // see if all entities have the same constant value
         // TODO: should be done somewhere else?
         let constValues = new Set<DataValue>();
+        let isConst = false;
         for (let e of this.entities) {
             let constVal = e.consts[mksymbol(component, fieldName)];
+            if (constVal !== undefined) isConst = true;
             constValues.add(constVal); // constVal === undefined is allowed
         }
         // can't write to constant
-        if (constValues.size > 0 && canwrite)
+        if (isConst && canWrite)
             throw new ECSError(`can't write to constant field ${fieldName}`, action);
         // is it a constant?
         if (constValues.size == 1) {
@@ -781,6 +791,7 @@ export class EntityScope implements SourceLocated {
     childScopes: EntityScope[] = [];
     systems: System[] = [];
     entities: Entity[] = [];
+    events: Action[] = [];
     bss = new DataSegment();
     rodata = new DataSegment();
     code = new CodeSegment();
@@ -813,6 +824,12 @@ export class EntityScope implements SourceLocated {
         this.entities.push(entity);
         return entity;
     }
+    addUsingSystem(system: System) {
+        this.systems.push(system);
+    }
+    addAction(action: Action) {
+        this.events.push(action);
+    }
     *iterateEntityFields(entities: Entity[]) {
         for (let i = 0; i < entities.length; i++) {
             let e = entities[i];
@@ -1057,11 +1074,18 @@ export class EntityScope implements SourceLocated {
         return symbol;
     }
     analyzeEntities() {
+        this.buildLocalSystem();
         this.buildSegments();
         this.allocateSegment(this.bss, false);
         this.allocateSegment(this.rodata, true);
         this.allocateROData(this.rodata);
     }
+    buildLocalSystem() {
+        if (this.events.length) {
+            let sys : System = { name: this.name, actions: this.events };
+            this.addUsingSystem(this.em.defineSystem(sys));
+        }
+    }
     generateCode() {
         this.tempOffset = this.maxTempBytes = 0;
         // TODO: main scope?