diff --git a/include/boost/mp11/detail/mp_is_list.hpp b/include/boost/mp11/detail/mp_is_list.hpp
new file mode 100644
index 0000000..25b378b
--- /dev/null
+++ b/include/boost/mp11/detail/mp_is_list.hpp
@@ -0,0 +1,39 @@
+#ifndef BOOST_MP11_DETAIL_MP_IS_LIST_HPP_INCLUDED
+#define BOOST_MP11_DETAIL_MP_IS_LIST_HPP_INCLUDED
+
+// Copyright 2015-2019 Peter Dimov.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/integral.hpp>
+
+namespace boost
+{
+namespace mp11
+{
+
+// mp_is_list<L>
+namespace detail
+{
+
+template<class L> struct mp_is_list_impl
+{
+    using type = mp_false;
+};
+
+template<template<class...> class L, class... T> struct mp_is_list_impl<L<T...>>
+{
+    using type = mp_true;
+};
+
+} // namespace detail
+
+template<class L> using mp_is_list = typename detail::mp_is_list_impl<L>::type;
+
+} // namespace mp11
+} // namespace boost
+
+#endif // #ifndef BOOST_MP11_DETAIL_MP_IS_LIST_HPP_INCLUDED
diff --git a/include/boost/mp11/list.hpp b/include/boost/mp11/list.hpp
index b764175..b56945a 100644
--- a/include/boost/mp11/list.hpp
+++ b/include/boost/mp11/list.hpp
@@ -10,6 +10,7 @@
 
 #include <boost/mp11/integral.hpp>
 #include <boost/mp11/detail/mp_list.hpp>
+#include <boost/mp11/detail/mp_is_list.hpp>
 #include <boost/mp11/detail/mp_append.hpp>
 #include <boost/mp11/detail/config.hpp>
 #include <type_traits>
@@ -23,22 +24,7 @@ namespace mp11
 template<class T, T... I> using mp_list_c = mp_list<std::integral_constant<T, I>...>;
 
 // mp_is_list<L>
-namespace detail
-{
-
-template<class L> struct mp_is_list_impl
-{
-    using type = mp_false;
-};
-
-template<template<class...> class L, class... T> struct mp_is_list_impl<L<T...>>
-{
-    using type = mp_true;
-};
-
-} // namespace detail
-
-template<class L> using mp_is_list = typename detail::mp_is_list_impl<L>::type;
+//   in detail/mp_is_list.hpp
 
 // mp_size<L>
 namespace detail