Add mp_is_set

include/boost/mp11/set.hpp

@@ -9,6 +9,7 @@
 //  http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/mp11/utility.hpp>
+#include <boost/mp11/detail/mp_list.hpp>
 #include <type_traits>
 
 namespace boost
@@ -78,6 +79,24 @@ template<template<class...> class L, class... U, class T1, class... T> struct mp
 
 template<class S, class... T> using mp_set_push_front = typename detail::mp_set_push_front_impl<S, T...>::type;
 
+// mp_is_set<S>
+namespace detail
+{
+
+template<class S> struct mp_is_set_impl
+{
+    using type = mp_false;
+};
+
+template<template<class...> class L, class... T> struct mp_is_set_impl<L<T...>>
+{
+    using type = mp_to_bool<std::is_same<mp_list<T...>, mp_set_push_back<mp_list<>, T...>>>;
+};
+
+} // namespace detail
+
+template<class S> using mp_is_set = typename detail::mp_is_set_impl<S>::type;
+
 } // namespace mp11
 } // namespace boost