diff --git a/include/xo/reader/define_xs.hpp b/include/xo/reader/define_xs.hpp
index b83ada71..e32d47a8 100644
--- a/include/xo/reader/define_xs.hpp
+++ b/include/xo/reader/define_xs.hpp
@@ -41,6 +41,7 @@ namespace xo {
             virtual void on_colon(exprstatestack * p_stack) override;
             virtual void on_semicolon(exprstatestack * p_stack,
                                       rp<Expression> * /*p_emit_expr*/) override;
+            virtual void on_singleassign(exprstatestack * p_stack) override;
 
         private:
             /**
diff --git a/src/reader/define_xs.cpp b/src/reader/define_xs.cpp
index d5513bf2..df468503 100644
--- a/src/reader/define_xs.cpp
+++ b/src/reader/define_xs.cpp
@@ -475,6 +475,32 @@ namespace xo {
                 assert(false);
             }
         }
+
+        void
+        define_xs::on_singleassign(exprstatestack * p_stack)
+        {
+            constexpr bool c_debug_flag = true;
+            scope log(XO_DEBUG(c_debug_flag));
+
+            constexpr const char * self_name = "exprstate::on_singleassign";
+
+            if (!this->admits_singleassign())
+            {
+                throw std::runtime_error(tostr(self_name,
+                                               ": unexpected equals for parsing state",
+                                               xtag("state", *this)));
+            }
+
+            if ((this->exs_type_ == exprstatetype::def_1)
+                || (this->exs_type_ == exprstatetype::def_3))
+            {
+                this->exs_type_ = exprstatetype::def_4;
+
+                p_stack->push_exprstate(exprstate::expect_rhs_expression());
+            } else {
+                assert(false);
+            }
+        }
     } /*namespace scm*/
 } /*namespace xo*/
 
diff --git a/src/reader/exprstate.cpp b/src/reader/exprstate.cpp
index 77a376c3..a0e9b0c7 100644
--- a/src/reader/exprstate.cpp
+++ b/src/reader/exprstate.cpp
@@ -658,7 +658,7 @@ namespace xo {
         }
 
         void
-        exprstate::on_singleassign(exprstatestack * p_stack) {
+        exprstate::on_singleassign(exprstatestack * /*p_stack*/) {
             constexpr bool c_debug_flag = true;
             scope log(XO_DEBUG(c_debug_flag));
 
@@ -671,15 +671,7 @@ namespace xo {
                                                xtag("state", *this)));
             }
 
-            if ((this->exs_type_ == exprstatetype::def_1)
-                || (this->exs_type_ == exprstatetype::def_3))
-            {
-                this->exs_type_ = exprstatetype::def_4;
-
-                p_stack->push_exprstate(exprstate::expect_rhs_expression());
-            } else {
-                assert(false);
-            }
+            assert(false);
         }
 
         void