diff --git a/README.md b/README.md
index c286e5b1e..1f0e64f47 100644
--- a/README.md
+++ b/README.md
@@ -91,7 +91,7 @@ Procedures for each standard are provided by the following R7RS-style libraries:
 cmake -B build -DCMAKE_BUILD_TYPE=Release
 cd build
 make package
-sudo apt install build/meevax_0.5.72_amd64.deb
+sudo apt install build/meevax_0.5.73_amd64.deb
 ```
 
 or
@@ -123,9 +123,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.5.72.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.5.73.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.5.72_amd64.deb`
+| `package`   | Generate debian package `meevax_0.5.73_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage
diff --git a/VERSION b/VERSION
index 873f2efa2..ef49227c1 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.5.72
+0.5.73
diff --git a/include/meevax/kernel/pair.hpp b/include/meevax/kernel/pair.hpp
index 59dde7cf9..03a5f199d 100644
--- a/include/meevax/kernel/pair.hpp
+++ b/include/meevax/kernel/pair.hpp
@@ -18,7 +18,7 @@
 #define INCLUDED_MEEVAX_KERNEL_PAIR_HPP
 
 #include <meevax/kernel/instruction.hpp>
-#include <meevax/memory/heterogeneous_pointer.hpp>
+#include <meevax/memory/gc_pointer.hpp>
 
 namespace meevax
 {
@@ -26,20 +26,20 @@ inline namespace kernel
 {
   struct pair;
 
-  using object = heterogeneous_pointer<gc_pointer, pair, bool, std::int32_t, std::uint32_t, float, instruction>;
+  using object = gc_pointer<pair, bool, std::int32_t, std::uint32_t, float, instruction>;
 
   using let = object;
 
   let extern unit;
 
   template <typename T, typename... Ts>
-  auto make(Ts&&... xs)
+  auto make(Ts&&... xs) -> object
   {
     return object::allocate<T>(std::forward<decltype(xs)>(xs)...);
   }
 
   template <typename T>
-  auto make(T&& x)
+  auto make(T&& x) -> object
   {
     return object::allocate<std::decay_t<T>>(std::forward<decltype(x)>(x));
   }
diff --git a/include/meevax/memory/gc_pointer.hpp b/include/meevax/memory/gc_pointer.hpp
index f377615d2..7b527807d 100644
--- a/include/meevax/memory/gc_pointer.hpp
+++ b/include/meevax/memory/gc_pointer.hpp
@@ -18,6 +18,7 @@
 #define INCLUDED_MEEVAX_MEMORY_GC_POINTER_HPP
 
 #include <meevax/memory/collector.hpp>
+#include <meevax/memory/heterogeneous_pointer.hpp>
 #include <meevax/memory/nan_boxing_pointer.hpp>
 
 namespace meevax
@@ -25,21 +26,28 @@ namespace meevax
 inline namespace memory
 {
   template <typename... Ts>
-  struct gc_pointer : public nan_boxing_pointer<Ts...>
+  struct gc_pointer : public heterogeneous_pointer<nan_boxing_pointer, Ts...>
                     , private collector::registration
   {
-    explicit constexpr gc_pointer(std::nullptr_t = nullptr)
+    using pointer = heterogeneous_pointer<nan_boxing_pointer, Ts...>;
+
+    gc_pointer(gc_pointer const& gcp)
+      : pointer { gcp }
+      , collector::registration { gcp.header }
     {}
 
     template <typename T, REQUIRES(std::is_scalar<T>)>
     explicit gc_pointer(T const& datum)
-      : nan_boxing_pointer<Ts...> { datum }
-      , collector::registration { locate(nan_boxing_pointer<Ts...>::get()) }
+      : pointer { datum }
+      , collector::registration { locate(pointer::get()) }
     {}
 
-    explicit gc_pointer(gc_pointer const& gcp)
-      : nan_boxing_pointer<Ts...> { gcp }
-      , collector::registration { gcp.header }
+    gc_pointer(pointer const& p)
+      : pointer { p }
+      , collector::registration { locate(pointer::get()) }
+    {}
+
+    gc_pointer(std::nullptr_t = nullptr)
     {}
 
     auto operator =(gc_pointer const& gcp) -> auto &
@@ -50,13 +58,13 @@ inline namespace memory
 
     auto reset(gc_pointer const& gcp) -> void
     {
-      nan_boxing_pointer<Ts...>::reset(gcp);
+      pointer::reset(gcp);
       collector::registration::reset(gcp.header);
     }
 
     auto reset(std::nullptr_t = nullptr) -> void
     {
-      nan_boxing_pointer<Ts...>::reset();
+      pointer::reset();
       collector::registration::reset();
     }
   };
diff --git a/include/meevax/memory/heterogeneous_pointer.hpp b/include/meevax/memory/heterogeneous_pointer.hpp
index 36d7d6e15..845822e8a 100644
--- a/include/meevax/memory/heterogeneous_pointer.hpp
+++ b/include/meevax/memory/heterogeneous_pointer.hpp
@@ -24,6 +24,7 @@
 #include <meevax/memory/gc_pointer.hpp>
 #include <meevax/type_traits/is_equality_comparable.hpp>
 #include <meevax/type_traits/is_output_streamable.hpp>
+#include <meevax/type_traits/requires.hpp>
 #include <meevax/utility/combination.hpp>
 #include <meevax/utility/debug.hpp>
 #include <meevax/utility/demangle.hpp>