diff --git a/detail/default_eval.hpp b/detail/default_eval.hpp
index 26a791f..22f9cf7 100644
--- a/detail/default_eval.hpp
+++ b/detail/default_eval.hpp
@@ -156,7 +156,7 @@ namespace boost::proto17 {
         template <typename Expr, typename Tuple, typename Transform>
         auto transform_nonterminal (Expr const & expr, Tuple && tuple, Transform && transform);
 
-        template <typename Expr, typename Transform, typename = std::void_t<>>
+        template <typename Expr, typename Transform, expr_arity Arity, typename = std::void_t<>>
         struct default_transform_expression
         {
             auto operator() (Expr && expr, Transform && transform)
@@ -164,7 +164,11 @@ namespace boost::proto17 {
                 constexpr expr_kind kind = remove_cv_ref_t<Expr>::kind;
                 if constexpr (kind == expr_kind::expr_ref) {
                     decltype(auto) ref = ::boost::proto17::value(expr);
-                    default_transform_expression<decltype(ref), Transform> transformer;
+                    default_transform_expression<
+                        decltype(ref),
+                        Transform,
+                        detail::arity_of<kind>()
+                    > transformer;
                     return transformer(ref, static_cast<Transform &&>(transform));
                 } else if constexpr (kind == expr_kind::terminal || kind == expr_kind::placeholder) {
                     return static_cast<Expr &&>(expr);
@@ -186,10 +190,11 @@ namespace boost::proto17 {
             }
         };
 
-        template <typename Expr, typename Transform>
+        template <typename Expr, typename Transform, expr_arity Arity>
         struct default_transform_expression<
             Expr,
             Transform,
+            Arity,
             std::void_t<decltype(std::declval<Transform>()(std::declval<Expr>()))>
         >
         {
@@ -213,7 +218,7 @@ namespace boost::proto17 {
                 static_cast<Tuple &&>(tuple),
                 [&transform](auto && element) {
                     using element_t = decltype(element);
-                    default_transform_expression<element_t, Transform> transformer;
+                    default_transform_expression<element_t, Transform, detail::arity_of<Expr::kind>()> transformer;
                     return transformer(
                         static_cast<element_t &&>(element),
                         static_cast<Transform &&>(transform)
diff --git a/detail/expression.hpp b/detail/expression.hpp
index a7a020e..019ef48 100644
--- a/detail/expression.hpp
+++ b/detail/expression.hpp
@@ -4,6 +4,7 @@
 #include "../expression_fwd.hpp"
 
 #include <boost/hana/tuple.hpp>
+#include <boost/hana/size.hpp>
 
 #include <type_traits>
 
@@ -191,6 +192,142 @@ namespace boost::proto17 {
             U
         >::type;
 
+        enum class expr_arity {
+            invalid,
+            one,
+            two,
+            n
+        };
+
+        template <expr_kind Kind>
+        constexpr expr_arity arity_of ()
+        {
+            switch (Kind) {
+            case expr_kind::expr_ref:
+
+            case expr_kind::terminal:
+            case expr_kind::placeholder:
+
+            // unary
+            case expr_kind::unary_plus: // +
+            case expr_kind::negate: // -
+            case expr_kind::dereference: // *
+            case expr_kind::complement: // ~
+            case expr_kind::address_of: // &
+            case expr_kind::logical_not: // !
+            case expr_kind::pre_inc: // ++
+            case expr_kind::pre_dec: // --
+            case expr_kind::post_inc: // ++(int)
+            case expr_kind::post_dec: // --(int)
+                return expr_arity::one;
+
+            // binary
+            case expr_kind::shift_left: // <<
+            case expr_kind::shift_right: // >>
+            case expr_kind::multiplies: // *
+            case expr_kind::divides: // /
+            case expr_kind::modulus: // %
+            case expr_kind::plus: // +
+            case expr_kind::minus: // -
+            case expr_kind::less: // <
+            case expr_kind::greater: // >
+            case expr_kind::less_equal: // <=
+            case expr_kind::greater_equal: // >=
+            case expr_kind::equal_to: // ==
+            case expr_kind::not_equal_to: // !=
+            case expr_kind::logical_or: // ||
+            case expr_kind::logical_and: // &&
+            case expr_kind::bitwise_and: // &
+            case expr_kind::bitwise_or: // |
+            case expr_kind::bitwise_xor: // ^
+            case expr_kind::comma: // :
+            case expr_kind::mem_ptr: // ->*
+            case expr_kind::assign: // =
+            case expr_kind::shift_left_assign: // <<=
+            case expr_kind::shift_right_assign: // >>=
+            case expr_kind::multiplies_assign: // *=
+            case expr_kind::divides_assign: // /=
+            case expr_kind::modulus_assign: // %=
+            case expr_kind::plus_assign: // +=
+            case expr_kind::minus_assign: // -=
+            case expr_kind::bitwise_and_assign: // &=
+            case expr_kind::bitwise_or_assign: // |=
+            case expr_kind::bitwise_xor_assign: // ^=
+            case expr_kind::subscript: // []
+                return expr_arity::two;
+
+            // n-ary
+            case expr_kind::call: // ()
+                return expr_arity::n;
+
+            default:
+                return expr_arity::invalid;
+            }
+        }
+
+        template <expr_kind Kind>
+        constexpr auto tag_for ()
+        {
+#define CASE(x) if constexpr (Kind == expr_kind::x) return x ## _tag{}
+
+            CASE(expr_ref);
+
+            CASE(terminal);
+            CASE(placeholder);
+
+            // unary
+            CASE(unary_plus); // +
+            CASE(negate); // -
+            CASE(dereference); // *
+            CASE(complement); // ~
+            CASE(address_of); // &
+            CASE(logical_not); // !
+            CASE(pre_inc); // ++
+            CASE(pre_dec); // --
+            CASE(post_inc); // ++(int)
+            CASE(post_dec); // --(int)
+
+            // binary
+            CASE(shift_left); // <<
+            CASE(shift_right); // >>
+            CASE(multiplies); // *
+            CASE(divides); // /
+            CASE(modulus); // %
+            CASE(plus); // +
+            CASE(minus); // -
+            CASE(less); // <
+            CASE(greater); // >
+            CASE(less_equal); // <=
+            CASE(greater_equal); // >=
+            CASE(equal_to); // ==
+            CASE(not_equal_to); // !=
+            CASE(logical_or); // ||
+            CASE(logical_and); // &&
+            CASE(bitwise_and); // &
+            CASE(bitwise_or); // |
+            CASE(bitwise_xor); // ^
+            CASE(comma); // );
+            CASE(mem_ptr); // ->*
+            CASE(assign); // =
+            CASE(shift_left_assign); // <<=
+            CASE(shift_right_assign); // >>=
+            CASE(multiplies_assign); // *=
+            CASE(divides_assign); // /=
+            CASE(modulus_assign); // %=
+            CASE(plus_assign); // +=
+            CASE(minus_assign); // -=
+            CASE(bitwise_and_assign); // &=
+            CASE(bitwise_or_assign); // |=
+            CASE(bitwise_xor_assign); // ^=
+            CASE(subscript); // []
+
+            // n-ary
+            CASE(call); // ()
+
+#undef CASE
+    };
+
+
     }
 
 }
diff --git a/expression.hpp b/expression.hpp
index dd7d322..e49cdeb 100644
--- a/expression.hpp
+++ b/expression.hpp
@@ -295,7 +295,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 1UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::one,
             "value() is only defined for unary expressions."
         );
         if constexpr (Expr::kind == expr_kind::expr_ref) {
@@ -310,7 +310,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 1UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::one,
             "value() is only defined for unary expressions."
         );
         if constexpr (Expr::kind == expr_kind::expr_ref) {
@@ -325,7 +325,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 1UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::one,
             "value() is only defined for unary expressions."
         );
         if constexpr (Expr::kind == expr_kind::expr_ref) {
@@ -340,7 +340,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return expr.elements[0_c];
@@ -351,7 +351,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return expr.elements[0_c];
@@ -362,7 +362,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return std::move(expr.elements)[0_c];
@@ -373,7 +373,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return expr.elements[1_c];
@@ -384,7 +384,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return expr.elements[1_c];
@@ -395,7 +395,7 @@ namespace boost::proto17 {
     {
         using namespace hana::literals;
         static_assert(
-            decltype(hana::size(expr.elements))::value == 2UL,
+            detail::arity_of<Expr::kind>() == detail::expr_arity::two,
             "left() and right() are only defined for binary expressions."
         );
         return std::move(expr.elements)[1_c];
@@ -516,7 +516,8 @@ namespace boost::proto17 {
     template <typename Expr, typename Transform>
     auto transform (Expr && expr, Transform && transform)
     {
-        return detail::default_transform_expression<Expr, Transform>{}(
+        constexpr expr_kind kind = detail::remove_cv_ref_t<Expr>::kind;
+        return detail::default_transform_expression<Expr, Transform, detail::arity_of<kind>()>{}(
             static_cast<Expr &&>(expr),
             static_cast<Transform &&>(transform)
         );
diff --git a/expression_fwd.hpp b/expression_fwd.hpp
index 3eb3da1..1d1cbff 100644
--- a/expression_fwd.hpp
+++ b/expression_fwd.hpp
@@ -120,6 +120,62 @@ namespace boost::proto17 {
 
     }
 
+    // Tag types -- one per expr_kind:
+
+    struct expr_ref_tag {}; // TODO: Needed?
+
+    struct terminal_tag {};
+    struct placeholder_tag {};
+
+    // unary
+    struct unary_plus_tag {}; // +
+    struct negate_tag {}; // -
+    struct dereference_tag {}; // *
+    struct complement_tag {}; // ~
+    struct address_of_tag {}; // &
+    struct logical_not_tag {}; // !
+    struct pre_inc_tag {}; // ++
+    struct pre_dec_tag {}; // --
+    struct post_inc_tag {}; // ++(int)
+    struct post_dec_tag {}; // --(int)
+
+    // binary
+    struct shift_left_tag {}; // <<
+    struct shift_right_tag {}; // >>
+    struct multiplies_tag {}; // *
+    struct divides_tag {}; // /
+    struct modulus_tag {}; // %
+    struct plus_tag {}; // +
+    struct minus_tag {}; // -
+    struct less_tag {}; // <
+    struct greater_tag {}; // >
+    struct less_equal_tag {}; // <=
+    struct greater_equal_tag {}; // >=
+    struct equal_to_tag {}; // ==
+    struct not_equal_to_tag {}; // !=
+    struct logical_or_tag {}; // ||
+    struct logical_and_tag {}; // &&
+    struct bitwise_and_tag {}; // &
+    struct bitwise_or_tag {}; // |
+    struct bitwise_xor_tag {}; // ^
+    struct comma_tag {}; // _tag {};
+    struct mem_ptr_tag {}; // ->*
+    struct assign_tag {}; // =
+    struct shift_left_assign_tag {}; // <<=
+    struct shift_right_assign_tag {}; // >>=
+    struct multiplies_assign_tag {}; // *=
+    struct divides_assign_tag {}; // /=
+    struct modulus_assign_tag {}; // %=
+    struct plus_assign_tag {}; // +=
+    struct minus_assign_tag {}; // -=
+    struct bitwise_and_assign_tag {}; // &=
+    struct bitwise_or_assign_tag {}; // |=
+    struct bitwise_xor_assign_tag {}; // ^=
+    struct subscript_tag {}; // []
+
+    // n-ary
+    struct call_tag {}; // ()
+
 }
 
 #endif