diff --git a/detail/expression.hpp b/detail/expression.hpp
new file mode 100644
index 0000000..ce352a6
--- /dev/null
+++ b/detail/expression.hpp
@@ -0,0 +1,77 @@
+#ifndef BOOST_PROTO17_DETAIL_EXPRESSION_HPP_INCLUDED
+#define BOOST_PROTO17_DETAIL_EXPRESSION_HPP_INCLUDED
+
+#include "../expression_fwd.hpp"
+
+#include <type_traits>
+
+
+namespace boost::proto17 {
+
+    namespace detail {
+
+        template <typename T>
+        struct partial_decay
+        {
+            using type = T;
+        };
+
+        template <typename T>
+        struct partial_decay<T[]> { using type = T *; };
+        template <typename T, std::size_t N>
+        struct partial_decay<T[N]> { using type = T *; };
+
+        template <typename T>
+        struct partial_decay<T(&)[]> { using type = T *; };
+        template <typename T, std::size_t N>
+        struct partial_decay<T(&)[N]> { using type = T *; };
+
+        template <typename R, typename ...A>
+        struct partial_decay<R(A...)> { using type = R(*)(A...); };
+        template <typename R, typename ...A>
+        struct partial_decay<R(A..., ...)> { using type = R(*)(A..., ...); };
+
+        template <typename T,
+                  typename U = typename detail::partial_decay<T>::type,
+                  bool AddRValueRef = std::is_same_v<T, U> && !std::is_const_v<U>>
+        struct rhs_value_type_phase_1;
+
+        template <typename T, typename U>
+        struct rhs_value_type_phase_1<T, U, true>
+        { using type = U &&; };
+
+        template <typename T, typename U>
+        struct rhs_value_type_phase_1<T, U, false>
+        { using type = U; };
+
+        template <typename ...T>
+        struct is_expr
+        { static bool const value = false; };
+
+        template <expr_kind Kind, typename ...T>
+        struct is_expr<expression<Kind, T...>>
+        { static bool const value = true; };
+
+        template <typename T,
+                  typename U = typename rhs_value_type_phase_1<T>::type,
+                  bool RemoveRefs = std::is_rvalue_reference_v<U>,
+                  bool IsExpr = is_expr<std::decay_t<T>>::value>
+        struct rhs_type;
+
+        template <typename T, typename U, bool RemoveRefs>
+        struct rhs_type<T, U, RemoveRefs, true>
+        { using type = std::remove_cv_t<std::remove_reference_t<T>>; };
+
+        template <typename T, typename U>
+        struct rhs_type<T, U, true, false>
+        { using type = terminal<std::remove_reference_t<U>>; };
+
+        template <typename T, typename U>
+        struct rhs_type<T, U, false, false>
+        { using type = terminal<U>; };
+
+    }
+
+}
+
+#endif
diff --git a/expression_fwd.hpp b/expression_fwd.hpp
index 11d92c0..7828c18 100644
--- a/expression_fwd.hpp
+++ b/expression_fwd.hpp
@@ -1,6 +1,8 @@
 #ifndef BOOST_PROTO17_EXPRESSION_FWD_HPP_INCLUDED
 #define BOOST_PROTO17_EXPRESSION_FWD_HPP_INCLUDED
 
+#include <boost/hana/fwd/integral_constant.hpp>
+
 
 namespace boost::proto17 {
 
@@ -17,6 +19,12 @@ namespace boost::proto17 {
     template <expr_kind Kind, typename ...T>
     struct expression;
 
+    template <typename T>
+    using terminal = expression<expr_kind::terminal, T>;
+
+    template <long long I>
+    using placeholder = expression<expr_kind::placeholder, hana::llong<I>>;
+
 }
 
 #endif
diff --git a/operators.hpp b/operators.hpp
index 8bba6ef..826904c 100644
--- a/operators.hpp
+++ b/operators.hpp
@@ -1,8 +1,6 @@
 #ifndef BOOST_PROTO17_OPERATORS_HPP_INCLUDED
 #define BOOST_PROTO17_OPERATORS_HPP_INCLUDED
 
-#include "expression_fwd.hpp"
-
 
 namespace boost::proto17 {
 
diff --git a/sketch.cpp b/sketch.cpp
index 591f607..02704fa 100644
--- a/sketch.cpp
+++ b/sketch.cpp
@@ -1,6 +1,7 @@
 #include "expression_fwd.hpp"
 
 #include "operators.hpp"
+#include "detail/expression.hpp"
 
 #define BOOST_PROTO17_STREAM_OPERATORS // TODO: For testing.
 #include "print.hpp"
@@ -18,71 +19,8 @@
 
 namespace boost::proto17 {
 
-    template <typename T>
-    using terminal = expression<expr_kind::terminal, T>;
-
     namespace detail {
 
-        template <typename T>
-        struct partial_decay
-        {
-            using type = T;
-        };
-
-        template <typename T>
-        struct partial_decay<T[]> { using type = T *; };
-        template <typename T, std::size_t N>
-        struct partial_decay<T[N]> { using type = T *; };
-
-        template <typename T>
-        struct partial_decay<T(&)[]> { using type = T *; };
-        template <typename T, std::size_t N>
-        struct partial_decay<T(&)[N]> { using type = T *; };
-
-        template <typename R, typename ...A>
-        struct partial_decay<R(A...)> { using type = R(*)(A...); };
-        template <typename R, typename ...A>
-        struct partial_decay<R(A..., ...)> { using type = R(*)(A..., ...); };
-
-        template <typename T,
-                  typename U = typename detail::partial_decay<T>::type,
-                  bool AddRValueRef = std::is_same_v<T, U> && !std::is_const_v<U>>
-        struct rhs_value_type_phase_1;
-
-        template <typename T, typename U>
-        struct rhs_value_type_phase_1<T, U, true>
-        { using type = U &&; };
-
-        template <typename T, typename U>
-        struct rhs_value_type_phase_1<T, U, false>
-        { using type = U; };
-
-        template <typename ...T>
-        struct is_expr
-        { static bool const value = false; };
-
-        template <expr_kind Kind, typename ...T>
-        struct is_expr<expression<Kind, T...>>
-        { static bool const value = true; };
-
-        template <typename T,
-                  typename U = typename rhs_value_type_phase_1<T>::type,
-                  bool RemoveRefs = std::is_rvalue_reference_v<U>,
-                  bool IsExpr = is_expr<std::decay_t<T>>::value>
-        struct rhs_type;
-
-        template <typename T, typename U, bool RemoveRefs>
-        struct rhs_type<T, U, RemoveRefs, true>
-        { using type = std::remove_cv_t<std::remove_reference_t<T>>; };
-
-        template <typename T, typename U>
-        struct rhs_type<T, U, true, false>
-        { using type = terminal<std::remove_reference_t<U>>; };
-
-        template <typename T, typename U>
-        struct rhs_type<T, U, false, false>
-        { using type = terminal<U>; };
-
         template <typename Tuple, expr_kind Kind, typename ...T>
         auto default_eval_expr (expression<Kind, T...> const & expr, Tuple && tuple)
         {
@@ -194,9 +132,6 @@ namespace boost::proto17 {
         }
     };
 
-    template <long long I>
-    using placeholder = expression<expr_kind::placeholder, hana::llong<I>>;
-
     namespace literals {
 
         template <char ...c>