diff --git a/README.md b/README.md
index 9fa31ef64..10dc6bf62 100644
--- a/README.md
+++ b/README.md
@@ -99,7 +99,7 @@ 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_0.5.30_amd64.deb
+sudo apt install build/meevax_0.5.31_amd64.deb
 ```
 
 or
@@ -131,9 +131,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.5.30.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.5.31.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.5.30_amd64.deb`
+| `package`   | Generate debian package `meevax_0.5.31_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage
diff --git a/VERSION b/VERSION
index 9dad0d549..674bf1d80 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.5.30
+0.5.31
diff --git a/example/example.cpp b/example/example.cpp
index 9e7766003..22d932e56 100644
--- a/example/example.cpp
+++ b/example/example.cpp
@@ -4,12 +4,12 @@ using namespace meevax; // NOTE: DIRTY HACK
 
 extern "C"
 {
-  auto arity(object const& xs)
+  auto arity(object & xs)
   {
     return make<exact_integer>(length(xs));
   }
 
-  auto dummy_procedure(object const& xs)
+  auto dummy_procedure(object & xs)
   {
     std::cout << "\n; calling C++ function." << std::endl;
 
@@ -45,17 +45,17 @@ extern "C"
     }
   };
 
-  auto make_hoge(object const& xs)
+  auto make_hoge(object & xs)
   {
     return make<hoge>(xs[0].as<exact_integer>());
   }
 
-  auto is_hoge(object const& xs)
+  auto is_hoge(object & xs)
   {
     return xs[0].is<hoge>() ? t : f;
   }
 
-  auto hoge_value(object const& xs)
+  auto hoge_value(object & xs)
   {
     return make<exact_integer>(xs[0].as<hoge>().value);
   }
diff --git a/include/meevax/kernel/procedure.hpp b/include/meevax/kernel/procedure.hpp
index 8901af864..199256839 100644
--- a/include/meevax/kernel/procedure.hpp
+++ b/include/meevax/kernel/procedure.hpp
@@ -85,7 +85,7 @@ inline namespace kernel
     }
   };
 
-  using procedure_pointer = auto (*)(object const&) -> object;
+  using procedure_pointer = auto (*)(object &) -> object;
 
   auto dlopen(std::string const&) -> void *;