Make "at <target> { ... }" require the braces

packages/compiler/src/compiler/grammar.ts

@@ -320,7 +320,7 @@ export class SyndicateParser {
             o.body = [];
             return seq(atom('at'),
                        bind(o, 'target', this.expr()),
-                       this.statement(o.body));
+                       this.block(o.body));
         });
 
     // Principal: none