diff --git a/example/Makefile b/example/Makefile
index 3c0fc71..10237a9 100644
--- a/example/Makefile
+++ b/example/Makefile
@@ -9,10 +9,13 @@ CC = g++
 CFLAGS = -std=c++1y -g
 endif
 
-all: calc calc2
+all: calc calc2 calc3
 
 calc : calc.cc ../peglib.h
 	$(CC) -o calc $(CFLAGS) -I.. calc.cc
 
 calc2 : calc2.cc ../peglib.h
 	$(CC) -o calc2 $(CFLAGS) -I.. calc2.cc
+
+calc3 : calc3.cc ../peglib.h
+	$(CC) -o calc3 $(CFLAGS) -I.. calc3.cc
diff --git a/example/calc3.cc b/example/calc3.cc
new file mode 100644
index 0000000..4f19099
--- /dev/null
+++ b/example/calc3.cc
@@ -0,0 +1,123 @@
+//
+//  calc3.cc
+//
+//  Copyright (c) 2015 Yuji Hirose. All rights reserved.
+//  MIT License
+//
+
+#include <peglib.h>
+#include <iostream>
+#include <cstdlib>
+
+using namespace peglib;
+using namespace std;
+
+//
+//  PEG syntax:
+//
+//      EXPRESSION       <-  _ TERM (TERM_OPERATOR TERM)*
+//      TERM             <-  FACTOR (FACTOR_OPERATOR FACTOR)*
+//      FACTOR           <-  NUMBER / '(' _ EXPRESSION ')' _
+//      TERM_OPERATOR    <-  < [-+] > _
+//      FACTOR_OPERATOR  <-  < [/*] > _
+//      NUMBER           <-  < [0-9]+ > _
+//      ~_               <-  [ \t\r\n]*
+//
+
+struct ast_node
+{
+    virtual ~ast_node() = default;
+    virtual long eval() = 0;
+};
+
+struct ast_ope : public ast_node
+{
+    ast_ope(char ope, shared_ptr<ast_node> left, shared_ptr<ast_node> right)
+        : ope_(ope), left_(left), right_(right) {}
+
+    long eval() override {
+        switch (ope_) {
+            case '+': return left_->eval() + right_->eval();
+            case '-': return left_->eval() - right_->eval();
+            case '*': return left_->eval() * right_->eval();
+            case '/': return left_->eval() / right_->eval();
+        }
+        assert(false);
+        return 0;
+    };
+
+    static shared_ptr<ast_node> create(const SemanticValues& sv) {
+        if (sv.empty()) {
+            return nullptr;
+        }
+        return SemanticValues::reduce(
+            sv.begin() + 1,
+            sv.end(),
+            sv[0].val.get<shared_ptr<ast_node>>(),
+            [](shared_ptr<ast_node> r, SemanticValues::const_iterator i) {
+                auto ope = (i++)->val.get<char>();
+                auto nd = (i++)->val.get<shared_ptr<ast_node>>();
+                r = make_shared<ast_ope>(ope, r, nd);
+                return make_tuple(r, i);
+            });
+    }
+
+private:
+    char                 ope_;
+    shared_ptr<ast_node> left_;
+    shared_ptr<ast_node> right_;
+};
+
+struct ast_num : public ast_node
+{
+    ast_num(long num) : num_(num) {}
+
+    long eval() override { return num_; };
+
+    static shared_ptr<ast_node> create(const char* s, size_t l) {
+        return make_shared<ast_num>(atol(s));
+    }
+
+private:
+    long num_;
+};
+
+int main(int argc, const char** argv)
+{
+    if (argc < 2 || string("--help") == argv[1]) {
+        cout << "usage: calc3 [formula]" << endl;
+        return 1;
+    }
+
+    const char* s = argv[1];
+
+    const char* syntax =
+        "  EXPRESSION       <-  _ TERM (TERM_OPERATOR TERM)*      "
+        "  TERM             <-  FACTOR (FACTOR_OPERATOR FACTOR)*  "
+        "  FACTOR           <-  NUMBER / '(' _ EXPRESSION ')' _   "
+        "  TERM_OPERATOR    <-  < [-+] > _                        "
+        "  FACTOR_OPERATOR  <-  < [/*] > _                        "
+        "  NUMBER           <-  < [0-9]+ > _                      "
+        "  ~_               <-  [ \t\r\n]*                        "
+        ;
+
+    peg parser(syntax);
+
+    parser["EXPRESSION"]      = ast_ope::create;
+    parser["TERM"]            = ast_ope::create;
+    parser["TERM_OPERATOR"]   = [](const char* s, size_t l) { return *s; };
+    parser["FACTOR_OPERATOR"] = [](const char* s, size_t l) { return *s; };
+    parser["NUMBER"]          = ast_num::create;
+
+    shared_ptr<ast_node> ast;
+    if (parser.parse(s, ast)) {
+        cout << s << " = " << ast->eval() << endl;
+        return 0;
+    }
+
+    cout << "syntax error..." << endl;
+
+    return -1;
+}
+
+// vim: et ts=4 sw=4 cin cino={1s ff=unix
diff --git a/peglib.h b/peglib.h
index 9a25e47..56bf2d6 100644
--- a/peglib.h
+++ b/peglib.h
@@ -167,6 +167,8 @@ struct SemanticValues : protected std::vector<SemanticValue>
     SemanticValues() : s(nullptr), l(0) {}
 
     typedef SemanticValue T;
+    using std::vector<T>::iterator;
+    using std::vector<T>::const_iterator;
     using std::vector<T>::size;
     using std::vector<T>::empty;
     using std::vector<T>::assign;
@@ -185,6 +187,15 @@ struct SemanticValues : protected std::vector<SemanticValue>
     using std::vector<T>::erase;
     using std::vector<T>::clear;
     using std::vector<T>::swap;
+
+    template <typename T, typename U, typename V>
+    static U reduce(T i, T end, U val, V f){
+        if (i == end) {
+            return val;
+        }
+        std::tie(val, i) = f(val, i);
+        return reduce(i, end, val, f);
+    };
 };
 
 /*