diff --git a/include/xo/reader/exprstate.hpp b/include/xo/reader/exprstate.hpp
index f3b00d51..c21f453f 100644
--- a/include/xo/reader/exprstate.hpp
+++ b/include/xo/reader/exprstate.hpp
@@ -172,6 +172,9 @@ namespace xo {
             /** handle incoming '=' token **/
             virtual void on_singleassign_token(const token_type & tk,
                                                parserstatemachine * p_psm);
+            /** handle incoming ':=' token **/
+            virtual void on_assign_token(const token_type & tk,
+                                         parserstatemachine * p_psm);
             /** handle incoming '(' token **/
             virtual void on_leftparen_token(const token_type & tk,
                                             parserstatemachine * p_psm);
diff --git a/include/xo/reader/progress_xs.hpp b/include/xo/reader/progress_xs.hpp
index ce77966d..780f5232 100644
--- a/include/xo/reader/progress_xs.hpp
+++ b/include/xo/reader/progress_xs.hpp
@@ -97,7 +97,7 @@ namespace xo {
                                   bp<Expression> expr2,
                                   parserstatemachine * p_psm) const;
 
-            virtual const char * get_expect_str() const override;
+            virtual const char * get_expect_str() const final override;
 
             virtual void on_expr(bp<Expression> expr,
                                  parserstatemachine * p_psm) override;
@@ -116,6 +116,8 @@ namespace xo {
                                             parserstatemachine * p_psm) override;
             virtual void on_singleassign_token(const token_type & tk,
                                                parserstatemachine * p_psm) override;
+            virtual void on_assign_token(const token_type & tk,
+                                         parserstatemachine * p_psm) final override;
             virtual void on_leftparen_token(const token_type & tk,
                                             parserstatemachine * p_psm) override;
             virtual void on_rightparen_token(const token_type & tk,
@@ -129,7 +131,7 @@ namespace xo {
 
             /* entry point for an infix operator token */
             virtual void on_operator_token(const token_type & tk,
-                                           parserstatemachine * p_psm) override;
+                                           parserstatemachine * p_psm) final override;
 
             virtual void on_bool_token(const token_type & tk,
                                        parserstatemachine * p_psm) override;
diff --git a/src/reader/exprstate.cpp b/src/reader/exprstate.cpp
index 0a8b0d32..47e368d4 100644
--- a/src/reader/exprstate.cpp
+++ b/src/reader/exprstate.cpp
@@ -216,6 +216,20 @@ namespace xo {
             this->illegal_input_on_token(c_self_name, tk, exp, p_psm);
         }
 
+        void
+        exprstate::on_assign_token(const token_type & tk,
+                                   parserstatemachine * p_psm)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * c_self_name = "exprstate::on_assign_token";
+            const char * exp = get_expect_str();
+
+            this->illegal_input_on_token(c_self_name, tk, exp, p_psm);
+        }
+
+
         void
         exprstate::on_leftparen_token(const token_type & tk,
                                       parserstatemachine * p_psm)
@@ -445,6 +459,9 @@ namespace xo {
                 return;
 
             case tokentype::tk_assign:
+                this->on_assign_token(tk, p_psm);
+                return;
+
             case tokentype::tk_yields:
                 assert(false);
                 break;
diff --git a/src/reader/progress_xs.cpp b/src/reader/progress_xs.cpp
index c99aaaa6..d47c8cf9 100644
--- a/src/reader/progress_xs.cpp
+++ b/src/reader/progress_xs.cpp
@@ -434,6 +434,13 @@ namespace xo {
             this->illegal_input_on_token(c_self_name, tk, exp, p_psm);
         }
 
+        void
+        progress_xs::on_assign_token(const token_type & tk,
+                                     parserstatemachine * p_psm)
+        {
+            this->on_operator_token(tk, p_psm);
+        }
+
         void
         progress_xs::on_leftparen_token(const token_type & tk,
                                         parserstatemachine * p_psm)
@@ -573,6 +580,8 @@ namespace xo {
             optype
             tk2op(const tokentype & tktype) {
                 switch (tktype) {
+                case tokentype::tk_assign:
+                    return optype::op_assign;
                 case tokentype::tk_plus:
                     return optype::op_add;
                 case tokentype::tk_minus: