diff --git a/include/xo/reader/define_xs.hpp b/include/xo/reader/define_xs.hpp
index c872a58b..41bf90d4 100644
--- a/include/xo/reader/define_xs.hpp
+++ b/include/xo/reader/define_xs.hpp
@@ -20,6 +20,8 @@ namespace xo {
 
             static std::unique_ptr<define_xs> def_0(rp<DefineExprAccess> def_expr);
 
+            virtual bool admits_definition() const override;
+            // virtual void on_f64(..) override
             virtual void on_expr(ref::brw<Expression> expr,
                                  exprstatestack * p_stack,
                                  rp<Expression> * p_emit_expr) override;
diff --git a/src/reader/define_xs.cpp b/src/reader/define_xs.cpp
index 09148c9e..5da374ad 100644
--- a/src/reader/define_xs.cpp
+++ b/src/reader/define_xs.cpp
@@ -10,6 +10,43 @@ namespace xo {
                         def_expr)
         {}
 
+        bool
+        define_xs::admits_definition() const
+        {
+            switch (exs_type_) {
+            case exprstatetype::expect_toplevel_expression_sequence:
+                /* unreachable */
+                assert(false);
+                return false;
+
+            case exprstatetype::def_0:
+            case exprstatetype::def_1:
+            case exprstatetype::def_2:
+            case exprstatetype::def_3:
+            case exprstatetype::def_4:
+            case exprstatetype::def_5:
+                /* note for def_4:
+                 * rhs could certainly be a function body that contains
+                 * nested defines; but then immediately-enclosing-exprstate
+                 * would be a block
+                 */
+                return false;
+            case exprstatetype::lparen_0:
+            case exprstatetype::lparen_1:
+            case exprstatetype::expect_rhs_expression:
+            case exprstatetype::expect_symbol:
+            case exprstatetype::expect_type:
+            case exprstatetype::expr_progress:
+            case exprstatetype::invalid:
+            case exprstatetype::n_exprstatetype:
+                /* unreachable */
+                assert(false);
+                return false;
+            }
+
+            return false;
+        }
+
         void
         define_xs::on_expr(ref::brw<Expression> expr,
                            exprstatestack * /* p_stack */,
diff --git a/src/reader/parser.cpp b/src/reader/parser.cpp
index 8e0df1ef..e5700bdd 100644
--- a/src/reader/parser.cpp
+++ b/src/reader/parser.cpp
@@ -70,11 +70,7 @@ namespace xo {
             case exprstatetype::def_3:
             case exprstatetype::def_4:
             case exprstatetype::def_5:
-                /* note for def_4:
-                 * rhs could certainly be a function body that contains
-                 * nested defines; but then immediately-enclosing-exprstate
-                 * would be a block
-                 */
+                /* unreachable */
                 return false;
             case exprstatetype::lparen_0:
             case exprstatetype::lparen_1: