diff --git a/include/xo/reader/paren_xs.hpp b/include/xo/reader/paren_xs.hpp
index 65811607..dada4627 100644
--- a/include/xo/reader/paren_xs.hpp
+++ b/include/xo/reader/paren_xs.hpp
@@ -29,7 +29,7 @@ namespace xo {
 
             static const paren_xs * from(const exprstate * x) { return dynamic_cast<const paren_xs *>(x); }
 
-            static std::unique_ptr<paren_xs> lparen_0();
+            static void start(exprstatestack * p_stack);
 
             bool admits_f64() const;
             bool admits_rightparen() const;
@@ -68,6 +68,9 @@ namespace xo {
 
             virtual void print(std::ostream & os) const override;
 
+        private:
+            static std::unique_ptr<paren_xs> make();
+
         private:
             /**
              *  ( foo ... )
diff --git a/src/reader/expect_expr_xs.cpp b/src/reader/expect_expr_xs.cpp
index 9f27d2a0..8ecc2e5b 100644
--- a/src/reader/expect_expr_xs.cpp
+++ b/src/reader/expect_expr_xs.cpp
@@ -57,8 +57,7 @@ namespace xo {
             //constexpr const char * self_name = "exprstate::on_leftparen";
 
             /* push lparen_0 to remember to look for subsequent rightparen. */
-            p_stack->push_exprstate(paren_xs::lparen_0());
-            expect_expr_xs::start(p_stack);
+            paren_xs::start(p_stack);
         }
 
         void