diff --git a/include/boost/stl_interfaces/container_interface.hpp b/include/boost/stl_interfaces/container_interface.hpp
index c2ac852..feb5104 100644
--- a/include/boost/stl_interfaces/container_interface.hpp
+++ b/include/boost/stl_interfaces/container_interface.hpp
@@ -12,18 +12,8 @@
 namespace boost { namespace stl_interfaces {
 
     namespace detail {
-        template<typename T, typename = void>
-        struct has_reserve : std::false_type
-        {
-        };
-        template<typename T>
-        struct has_reserve<T, void_t<decltype(std::declval<T &>().reserve())>>
-            : std::true_type
-        {
-        };
-
         template<typename T, typename SizeType>
-        struct n_iter : boost::stl_interfaces::iterator_interface<
+        struct n_iter : iterator_interface<
                             n_iter<T, SizeType>,
                             std::random_access_iterator_tag,
                             T>
@@ -42,7 +32,7 @@ namespace boost { namespace stl_interfaces {
             }
 
         private:
-            friend boost::stl_interfaces::access;
+            friend access;
             constexpr T const *& base_reference() noexcept { return x_; }
             constexpr T const * base_reference() const noexcept { return x_; }
 
@@ -54,7 +44,10 @@ namespace boost { namespace stl_interfaces {
         constexpr auto make_n_iter(T const & x, SizeType n) noexcept(
             noexcept(n_iter<T, SizeType>(x, n)))
         {
-            return n_iter<T, SizeType>(x, SizeType(0));
+            using result_type = n_iter<T, SizeType>;
+            BOOST_STL_INTERFACES_STATIC_ASSERT_CONCEPT(
+                result_type, std::random_access_iterator)
+            return result_type(x, SizeType(0));
         }
         template<typename T, typename SizeType>
         constexpr auto make_n_iter_end(T const & x, SizeType n) noexcept(
diff --git a/include/boost/stl_interfaces/fwd.hpp b/include/boost/stl_interfaces/fwd.hpp
index 85d68a0..974adda 100644
--- a/include/boost/stl_interfaces/fwd.hpp
+++ b/include/boost/stl_interfaces/fwd.hpp
@@ -55,11 +55,9 @@ namespace boost { namespace stl_interfaces {
         template<typename Range, typename = void>
         struct sentinel;
         template<typename Range>
-        struct sentinel<
-            Range,
-            void_t<decltype(std::declval<Range &>().begin())>>
+        struct sentinel<Range, void_t<decltype(std::declval<Range &>().end())>>
         {
-            using type = decltype(std::declval<Range &>().begin());
+            using type = decltype(std::declval<Range &>().end());
         };
         template<typename Range>
         using sentinel_t = typename sentinel<Range>::type;
diff --git a/include/boost/stl_interfaces/reverse_iterator.hpp b/include/boost/stl_interfaces/reverse_iterator.hpp
index bd81bf5..7a65345 100644
--- a/include/boost/stl_interfaces/reverse_iterator.hpp
+++ b/include/boost/stl_interfaces/reverse_iterator.hpp
@@ -11,7 +11,6 @@
 
 namespace boost { namespace stl_interfaces {
 
-    // TODO: Tests.
     // TODO: Look in all the headers for ADL traps.
 
     /** This type is very similar to the C++20 version of
@@ -19,7 +18,7 @@ namespace boost { namespace stl_interfaces {
         proxy-friendly. */
     template<typename BidiIter>
     struct reverse_iterator
-        : boost::stl_interfaces::iterator_interface<
+        : iterator_interface<
               reverse_iterator<BidiIter>,
 #if 201703L < __cplusplus && defined(__cpp_lib_ranges)
               typename std::iterator_traits<BidiIter>::iterator_concept,
@@ -75,7 +74,7 @@ namespace boost { namespace stl_interfaces {
         }
 
     private:
-        friend boost::stl_interfaces::access;
+        friend access;
         constexpr BidiIter & base_reference() noexcept { return it_; }
         constexpr BidiIter const & base_reference() const noexcept
         {
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 0a110c5..0f467b7 100755
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -46,3 +46,4 @@ add_test_executable(forward)
 add_test_executable(bidirectional)
 add_test_executable(random_access)
 add_test_executable(reverse_iter)
+add_test_executable(detail)
diff --git a/test/detail.cpp b/test/detail.cpp
new file mode 100644
index 0000000..3aed466
--- /dev/null
+++ b/test/detail.cpp
@@ -0,0 +1,82 @@
+// Copyright (C) 2019 T. Zachary Laine
+//
+// 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/stl_interfaces/iterator_interface.hpp>
+#include <boost/stl_interfaces/view_interface.hpp>
+#include <boost/stl_interfaces/container_interface.hpp>
+
+#include <boost/mpl/assert.hpp>
+
+#include <gtest/gtest.h>
+
+#include <list>
+#include <vector>
+
+
+namespace detail = boost::stl_interfaces::detail;
+
+
+// iter_difference_t
+BOOST_MPL_ASSERT(
+    (std::is_same<detail::iter_difference_t<int *>, std::ptrdiff_t>));
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::iter_difference_t<std::vector<double>::iterator>,
+                  std::vector<double>::difference_type>));
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::iter_difference_t<std::list<double>::iterator>,
+                  std::list<double>::difference_type>));
+
+struct ridiculous_range
+{
+    int * begin() { return nullptr; }
+    double end() { return 1.3; }
+};
+
+// iterator_t
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::iterator_t<std::vector<double>>,
+                  std::vector<double>::iterator>));
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::iterator_t<std::list<double>>,
+                  std::list<double>::iterator>));
+BOOST_MPL_ASSERT((std::is_same<detail::iterator_t<ridiculous_range>, int *>));
+
+// sentinel_t
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::sentinel_t<std::vector<double>>,
+                  std::vector<double>::iterator>));
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::sentinel_t<std::list<double>>,
+                  std::list<double>::iterator>));
+BOOST_MPL_ASSERT((std::is_same<detail::sentinel_t<ridiculous_range>, double>));
+
+// range_difference_t
+BOOST_MPL_ASSERT(
+    (std::is_same<
+        detail::range_difference_t<std::vector<double>>,
+        std::iterator_traits<std::vector<double>::iterator>::difference_type>));
+BOOST_MPL_ASSERT(
+    (std::is_same<
+        detail::range_difference_t<std::list<double>>,
+        std::iterator_traits<std::list<double>::iterator>::difference_type>));
+BOOST_MPL_ASSERT((std::is_same<
+                  detail::range_difference_t<ridiculous_range>,
+                  std::ptrdiff_t>));
+
+// common_range
+static_assert(detail::common_range<std::vector<double>>::value, "");
+static_assert(detail::common_range<std::list<double>>::value, "");
+static_assert(!detail::common_range<ridiculous_range>::value, "");
+
+
+TEST(detail, n_iter)
+{
+    std::array<int, 5> ints = {{0, 1, 2, 3, 4}};
+    int const new_value = 6;
+    detail::n_iter<int, int> first = detail::make_n_iter(new_value, 3);
+    detail::n_iter<int, int> last = detail::make_n_iter_end(new_value, 3);
+    std::copy(first, last, &ints[1]);
+    EXPECT_EQ(ints, (std::array<int, 5>{0, 6, 6, 6, 4}));
+}