diff --git a/nac3core/irrt/irrt.cpp b/nac3core/irrt/irrt.cpp
index f086ba5a..c79fcd91 100644
--- a/nac3core/irrt/irrt.cpp
+++ b/nac3core/irrt/irrt.cpp
@@ -1,5 +1,6 @@
 #include <irrt/exception.hpp>
 #include <irrt/int_types.hpp>
+#include <irrt/list.hpp>
 #include <irrt/math_util.hpp>
 #include <irrt/ndarray/basic.hpp>
 #include <irrt/ndarray/def.hpp>
diff --git a/nac3core/irrt/irrt/list.hpp b/nac3core/irrt/irrt/list.hpp
new file mode 100644
index 00000000..25011f11
--- /dev/null
+++ b/nac3core/irrt/irrt/list.hpp
@@ -0,0 +1,19 @@
+#pragma once
+
+#include <irrt/int_types.hpp>
+#include <irrt/slice.hpp>
+
+namespace
+{
+/**
+ * @brief A list in NAC3.
+ *
+ * The `items` field is opaque. You must rely on external contexts to
+ * know how to interpret it.
+ */
+template <typename SizeT> struct List
+{
+    uint8_t *items;
+    SizeT len;
+};
+} // namespace
\ No newline at end of file