MlValue::is_string added

ml.h

@@ -136,17 +136,18 @@ public:
 
     std::vector<std::string> get_used_atoms();
 
-    // Is this a builtin function?
-    bool is_builtin();
-
     // Apply this as a function to a list of arguments in a given environment.
     MlValue apply(std::vector<MlValue> args, MlEnvironment &env);
 
     // Evaluate this value as lisp code.
     MlValue eval(MlEnvironment &env);
 
+    bool is_builtin() const;
+
     bool is_number() const;
 
+    bool is_string() const;
+
     // Get the boolean value of this value.
     bool as_bool() const;