diff --git a/include/xo/expression/LocalEnv.hpp b/include/xo/expression/LocalEnv.hpp
index 3106dc6c..ec3d4c81 100644
--- a/include/xo/expression/LocalEnv.hpp
+++ b/include/xo/expression/LocalEnv.hpp
@@ -35,6 +35,11 @@ namespace xo {
             int n_arg() const { return argv_.size(); }
             TypeDescr fn_arg(uint32_t i) const { return argv_[i]->valuetype(); }
 
+            /** report binding path for a formal parameter.
+             *  Returns sentinel if @p vname doesn't appear in @ref argv_
+             **/
+            binding_path lookup_local_binding(const std::string & vname) const;
+
             /** single-assign this environment's origin **/
             void assign_origin(Lambda * p) {
                 assert(origin_ == nullptr);
diff --git a/src/expression/LocalEnv.cpp b/src/expression/LocalEnv.cpp
index fb724aeb..8b834278 100644
--- a/src/expression/LocalEnv.cpp
+++ b/src/expression/LocalEnv.cpp
@@ -8,7 +8,7 @@
 namespace xo {
     namespace ast {
         binding_path
-        LocalEnv::lookup_binding(const std::string & vname) const
+        LocalEnv::lookup_local_binding(const std::string & vname) const
         {
             int j_slot = 0;
             for (const auto & arg : argv_) {
@@ -17,12 +17,25 @@ namespace xo {
                 ++j_slot;
             }
 
-            auto tmp = parent_env_->lookup_binding(vname);
+            return { -2 /*i_link: sentinel*/, 0 };
+        } /*lookup_local_binding*/
 
-            if (tmp.i_link_ == -1)
-                return tmp;
+        binding_path
+        LocalEnv::lookup_binding(const std::string & vname) const
+        {
+            {
+                auto local = this->lookup_local_binding(vname);
+
+                if (local.i_link_ == 0)
+                    return local;
+            }
+
+            auto free = parent_env_->lookup_binding(vname);
+
+            if (free.i_link_ == -1)
+                return free;
             else
-                return { tmp.i_link_ + 1, tmp.j_slot_ };
+                return { free.i_link_ + 1, free.j_slot_ };
         } /*lookup_binding*/
     } /*namespace ast*/
 } /*namespace xo*/