Add explanation

README.md

@@ -9,7 +9,7 @@ to reduce boilerplate code using lambda expression.
 
 To use jcombinator, Add the following lines to your pom.xml
 
-```
+```xml
 <dependencies>
     <dependency>
         <groupId>com.github.kmizu</groupId>
@@ -20,6 +20,66 @@ To use jcombinator, Add the following lines to your pom.xml
 </dependencies>
 ```
 
-## Usage
+### Usage
+
+See [JavaDoc](http://javadoc-badge.appsp    ot.com/com.github.kmizu/jcombinator/index.html#com.github.kmizu.jcombinator.package) and [Tests](https://github.com/kmizu/jcombinator/tree/releases%2F0.0.1/src/test/java/com/github/kmizu/jcombinator)
+
+### Example
+
+The following code is a parser of arithmetic expression:
+
+```java
+
+import com.github.kmizu.jcombinator.datatype.Function2;
+
+import static com.github.kmizu.jcombinator.Parser.*;
+import static com.github.kmizu.jcombinator.Functions.*;
+
+public class ArithmeticExpression {
+	private Rule<Integer> expression() {
+	    return rule(() ->
+			additive().cat(eof()).map(t -> t.extract((result, __) -> result))
+		);
+	}
+	private Rule<Integer> additive() {
+		return rule(() -> {
+			final Parser<Function2<Integer, Integer, Integer>> Q = string("+").map(op -> (Integer lhs, Integer rhs) -> lhs + rhs);
+			final Parser<Function2<Integer, Integer, Integer>> R = string("-").map(op -> (Integer lhs, Integer rhs) -> lhs - rhs);
+			return multitive().chain(Q.or(R));
+		});
+	}
+	private Rule<Integer> multitive() {
+		return rule(() -> {
+			final Parser<Function2<Integer, Integer, Integer>> Q = string("*").map(op -> (Integer lhs, Integer rhs) -> lhs * rhs);
+			final Parser<Function2<Integer, Integer, Integer>> R = string("/").map(op -> (Integer lhs, Integer rhs) -> lhs / rhs);
+			return primary().chain(Q.or(R));
+		});
+	}
+	private final Rule<Integer> primary() {
+		return rule(() ->
+			number().or((string("(").cat(expression())).cat(string(")")).map(t -> t.item1().item2()))
+		);
+	}
+	private final Rule<Integer> number() {
+		return rule(() ->
+			digit().many1().map(digits -> Integer.parseInt(join(digits, "")))
+		);
+	}
 
-See [JavaDoc](http://javadoc-badge.appspot.com/com.github.kmizu/jcombinator.svg?label=javadoc)](http://javadoc-badge.appsp    ot.com/com.github.kmizu/jcombinator/index.html#com.github.kmizu.jcombinator.package) and [Tests](https://github.com/kmizu/jcombinator/tree/releases%2F0.0.1/src/test/java/com/github/kmizu/jcombinator)
+    public void testExpression() {
+	    Parser<Integer> arithmetic = expression();
+    	arithmetic.invoke("100").onSuccess(s -> {
+    		assert ((Integer)100) == s.value();
+    	});
+    	arithmetic.invoke("100+200").onSuccess(s -> {
+    		assert ((Integer)300) == s.value();
+    	});
+    	arithmetic.invoke("(1+2)*(3+4)").onSuccess(s -> {
+    		assert ((Integer)21) == s.value();
+    	});
+        arithmetic.invoke("1+2*3+4").onSuccess(s -> {
+            assert ((Integer)11) == s.value();
+        });
+    }
+}
+```