diff --git a/README.md b/README.md
index a99f0e8..9b83e7b 100644
--- a/README.md
+++ b/README.md
@@ -198,9 +198,11 @@ It also supports named capture with the `$name<` ... `>` operator.
 peglib::match m;
 
 auto ret = peglib::peg_match(
-    "  ROOT      <-  _ ('[' $test< TAG_NAME > ']' _)*  "
-    "  TAG_NAME  <-  (!']' .)+                "
-    "  _         <-  [ \t]*                   ",
+    R"(
+        ROOT      <-  _ ('[' $test< TAG_NAME > ']' _)*
+        TAG_NAME  <-  (!']' .)+
+        _         <-  [ \t]*
+    )",
     " [tag1] [tag:2] [tag-3] ",
     m);
 
@@ -290,11 +292,6 @@ The following are available operators:
 | cap      | Capture character     |
 | usr      | User defiend parser   |
 
-Predicate control
------------------
-
-  * TODO
-
 Adjust definitions
 ------------------
 
@@ -344,7 +341,7 @@ Tested Compilers
 TODO
 ----
 
-  * Optimization of grammars
+  * Predicate control
   * Unicode support
 
 License
diff --git a/language/interpreter.hpp b/language/interpreter.hpp
index defceb8..3e0ba2b 100644
--- a/language/interpreter.hpp
+++ b/language/interpreter.hpp
@@ -17,23 +17,44 @@ struct Value
             std::string name;
             bool        mut;
         };
-        std::vector<Parameter>                            params;
+        std::vector<Parameter>                                  params;
         std::function<Value (std::shared_ptr<Environment> env)> eval;
     };
 
-    explicit Value() : type(Undefined) {}
+    Value() : type(Undefined) {
+        //std::cout << "Val::def ctor: " << std::endl;
+    }
+
+    Value(const Value& rhs) : type(rhs.type), v(rhs.v) {
+        //std::cout << "Val::copy ctor: " << *this << std::endl;
+    }
+
+    Value(Value&& rhs) : type(rhs.type), v(rhs.v) {
+        //std::cout << "Val::move ctor: " << *this << std::endl;
+    }
+
+    Value& operator=(const Value& rhs) {
+        if (this != &rhs) {
+            type = rhs.type;
+            v = rhs.v;
+            //std::cout << "Val::copy=: " << *this << std::endl;
+        }
+        return *this;
+    }
+
+    Value& operator=(Value&& rhs) {
+        type = rhs.type;
+        v = rhs.v;
+        //std::cout << "Val::move=: " << *this << std::endl;
+        return *this;
+    }
+
     explicit Value(bool b) : type(Bool), v(b) {}
     explicit Value(long l) : type(Long), v(l) {}
     explicit Value(std::string&& s) : type(String), v(s) {}
     explicit Value(ArrayValue&& a) : type(Array), v(a) {}
     explicit Value(FunctionValue&& f) : type(Function), v(f) {}
 
-    Value(const Value&) = default;
-    Value(Value&& rhs) : type(rhs.type), v(rhs.v) {}
-
-    Value& operator=(const Value&) = default;
-    Value& operator=(Value&& rhs) { type = rhs.type; v = rhs.v; return *this; }
-
     bool to_bool() const {
         switch (type) {
             case Bool: return v.get<bool>();
@@ -191,7 +212,7 @@ struct Environment
         return outer_ && outer_->has(s);
     }
 
-    Value get(const std::string& s) const {
+    const Value& get(const std::string& s) const {
         if (dic_.find(s) != dic_.end()) {
             return dic_.at(s).val;
         }
@@ -203,13 +224,13 @@ struct Environment
     }
 
     void assign(const std::string& s, const Value& val) {
-        assert(has(s));
         if (dic_.find(s) != dic_.end()) {
             auto& sym = dic_[s];
             if (!sym.mut) {
                 std::string msg = "immutable variable '" + s + "'...";
                 throw std::runtime_error(msg);
             }
+            //std::cout << "Env::assgin: " << s << std::endl;
             sym.val = val;
             return;
         }
@@ -222,7 +243,7 @@ struct Environment
     }
 
     void initialize(const std::string& s, const Value& val, bool mut) {
-        assert(!has(s));
+        //std::cout << "Env::initialize: " << s << std::endl;
         dic_[s] = Symbol{val, mut};
     }