diff --git a/crates/percival/src/codegen.rs b/crates/percival/src/codegen.rs
index f5b5780..05a5974 100644
--- a/crates/percival/src/codegen.rs
+++ b/crates/percival/src/codegen.rs
@@ -470,7 +470,7 @@ fn cmp_new_decls(ctx: &Context) -> (Context, String) {
     for result in Rc::clone(&ctx.results).iter() {
         let name = ctx.gensym(&format!("{}_new", result));
         decls.push(format!(
-            "let {} = {}.Set().asMutable();",
+            "const {} = {}.Set().asMutable();",
             name, VAR_IMMUTABLE,
         ));
         ctx = ctx.add(VarId::New(result.clone()), name);
@@ -533,7 +533,7 @@ fn cmp_rule_incremental(
 
     let goal = format!(
         "
-let {goal} = {imm}.Map({goal_obj});
+const {goal} = {imm}.Map({goal_obj});
 if (!{set}.includes({goal})) {new}.add({goal});
 ",
         goal = VAR_GOAL,
diff --git a/examples/purchased.txt b/examples/purchased.txt
new file mode 100644
index 0000000..7687830
--- /dev/null
+++ b/examples/purchased.txt
@@ -0,0 +1,5 @@
+all_purchased(buyer) :-
+  orders_unique(buyer),
+  num_purchased = count[1] { orders_unique(buyer) },
+  num_total = count[1] { food() },
+  `num_purchased === num_total`.
diff --git a/examples/rolling.txt b/examples/rolling.txt
new file mode 100644
index 0000000..2c76c0a
--- /dev/null
+++ b/examples/rolling.txt
@@ -0,0 +1,6 @@
+rolling(end_time, average) :-
+  data(time: end_time),
+  average = mean[x] {
+    data(time, x),
+    `end_time - 7 < time && time <= end_time`
+  }.