diff --git a/Readme.md b/Readme.md
index d39f20b..3dfecbf 100644
--- a/Readme.md
+++ b/Readme.md
@@ -37,7 +37,6 @@
   - get-env, set-env; set-env cannot be implemented in stdlib.lsp, because popen is in fact subshell
 - format (sprintf)
 - setq
-- mapcar (funcall, apply)
 - syntax highlighting do VS Code
 - add hash datatype
 
diff --git a/ml.cpp b/ml.cpp
index 565b289..3c2294d 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -1768,7 +1768,7 @@ void load_std_lib(MlEnvironment &env) {
 }
 
 // Does this environment, or its parent environment, have a variable?
-bool MlEnvironment::has(std::string name) const {
+bool MlEnvironment::has(const std::string &name) const {
 	// Find the value in the map
 	std::map<std::string, MlValue>::const_iterator itr = defs.find(name);
 	if (itr != defs.end())
diff --git a/ml.h b/ml.h
index 257aa5a..f5e5cd0 100644
--- a/ml.h
+++ b/ml.h
@@ -34,7 +34,7 @@ public:
     // have this atom in scope?
     // This is only used to determine which atoms to capture when
     // creating a lambda function.
-    bool has(std::string name) const;
+    bool has(const std::string &name) const;
 
     // Get the value associated with this name in this scope
     MlValue get(const std::string &name) const;