diff --git a/main.ts b/main.ts
index ef5ef47..2ceaec7 100644
--- a/main.ts
+++ b/main.ts
@@ -259,6 +259,13 @@ function occursIn(x: variable, e: expr): boolean {
       return isEqual(e.x, x);
     case "universe":
       return false;
+    case "pi":
+    case "lambda":
+      if (occursIn(x, e.abs.type)) return true;
+      if (!isEqual(e.abs.x, x)) return occursIn(x, e.abs.body);
+      return false;
+    case "app":
+      return occursIn(x, e.left) || occursIn(x, e.right);
   }
 }