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+++ b/README.md
@@ -1,147 +1,161 @@
-FlatDB
-===========
-
-FlatDB is a lua library that implements a serverless, zero-configuration, NoSQL database engine.<br>
-It provides a key-value storage system using plain Lua tables.
-
-When To Use FlatDB
-===========
-
-When you want to use SQLite to store data, just take a glance at FlatDB.<br>
-When Lua acts in your program as the major language or the embedded scripting language, just try using FlatDB.
-
-Usage
-==========
-Copy *flatdb.lua* file to your project or where your lua libraries stored.<br>
-Then write this in any Lua file where you want to use it:
-```lua
-local flatdb = require 'flatdb'
-```
-
+FlatDB
+===========
+
+FlatDB is a lua library that implements a serverless, zero-configuration, NoSQL database engine.<br>
+It provides a key-value storage system using plain Lua tables.
+
+When To Use FlatDB
+===========
+
+When you want to use SQLite to store data, just take a glance at FlatDB.<br>
+When Lua acts in your program as the major language or the embedded scripting language, just try using FlatDB.
+
+Concept
+==========
+
+|     FlatDB     |      Disk     |       Lua      |
+|:--------------:|:-------------:|:--------------:|
+| Database       | Directory     | Table          |
+| Book           | File          | Table          |
+| Key-value pair | File content  | Key-value pair |
+
+Keys and values can be all Lua types except coroutines, userdata, cdata and C functions.
+
+Usage
+==========
+
+Copy *flatdb.lua* file to your project or where your lua libraries stored.<br>
+Then write this in any Lua file where you want to use it:
+```lua
+local flatdb = require 'flatdb'
+```
+
 1. Bind a directory as a database
-
-    ```lua
-    local db = flatdb('./db')
-    ```
-
+
+    ```lua
+    local db = flatdb('./db')
+    ```
+
 2. Open or create a book
-
-    ```lua
-    if not db.book then
-    	db.book = {}
-    end
-    ```
-
+
+    ```lua
+    if not db.book then
+    	db.book = {}
+    end
+    ```
+
 3. Store key-value items
-
-    ```lua
-    db.book.key = 'value'
-    -- equivalent to db.book['key'] = 'value'
-    ```
-
+
+    ```lua
+    db.book.key = 'value'
+    -- equivalent to db.book['key'] = 'value'
+    ```
+
 4. Retrieve items
-
-    ```lua
-    print(db.book.key) -- prints 'value'
-    ```
-
+
+    ```lua
+    print(db.book.key) -- prints 'value'
+    ```
+
 5. Save to file
-
-    ```lua
-    db:save()
-    -- 'book' will be saved to './db/book'
-    ```
-
-More usage can be found in the *cli.lua*(a Redis-like command line interface example using FlatDB).
-
-Quick Look
-==========
-
-```lua
-local flatdb = require("flatdb")
-
--- open a directory as a database, 'db' is just a plain empty Lua table that can contain books
-local db = flatdb("./db")
-
--- open or create a book named "default", it also a plain empty Lua table where key-value data stored in
-if not db.default then
-	db.default = {}
-end
-
--- extend db methods for getting values from 'default' book
-flatdb.hack.get = function(db, key)
-	return db.default[key]
-end
-
--- extend db methods for setting values to 'default' book
-flatdb.hack.set = function(db, key, value)
-	db.default[key] = value
-end
-
--- extend db methods for watching new key
-flatdb.hack.guard = function(db, f)
-	setmetatable(db.default, {__newindex = f})
-end
-
--- get key-value data from 'default' book
-print(db:get("hello"))
-
--- set key-value data to 'default' book
-db:set("hello", "world")
-
--- get key-value data from 'default' book
-print(db:get("hello"))
-
--- set guard function
-db:guard(function(book, key, value)
-	print("CREATE KEY permission denied!")
-end)
-
--- try creating new key-value data to 'default' book
-db:set("key1", 1)
-db:set("key2", 2)
-
--- update an existing key-value item
-db:set("hello", "bye")
-
-print(db:get("key1")) -- prints nil
-print(db:get("key2")) -- prints nil
-print(db:get("hello")) -- prints 'bye'
-
--- store 'default' book to './db/default' file
-db:save()
-
-```
-
-API
-==========
-
-- **Functions**
-
-  - **flatdb(dir) --> db**
-
-      Bind a directory as a database, returns nil if 'dir' doesn't exists. Otherwise, it returns a 'db' obeject.
-
-  - **db:save([book])**
-
-      Save all books or the given book(if specified) contained in db to file. The 'book' argument is a string, the book's name.
-
-- **Tables**
-
-  - **flatdb.hack**
-
-      The 'hack' table contains db's methods. There is only one method 'save(db, book)' in it by default.
-      It is usually used to extend db methods.
-
-Dependencies
-=======
-
-- [pp](https://github.com/luapower/pp)
-- [lfs](http://keplerproject.github.io/luafilesystem/)
-
-All above libraries can be found in [LuaPower](https://luapower.com/).
-
-License
-=======
-
-FlatDB is distributed under the MIT license.
+
+    ```lua
+    db:save()
+    -- 'book' will be saved to './db/book'
+    ```
+
+More usage can be found in the *cli.lua*(a Redis-like command line interface example using FlatDB).
+
+Quick Look
+==========
+
+```lua
+local flatdb = require("flatdb")
+
+-- open a directory as a database
+-- 'db' is just a plain empty Lua table that can contain books
+local db = flatdb("./db")
+
+-- open or create a book named "default"
+-- it is also a plain empty Lua table where key-value pair stored in
+if not db.default then
+	db.default = {}
+end
+
+-- extend db methods for getting values from 'default' book
+flatdb.hack.get = function(db, key)
+	return db.default[key]
+end
+
+-- extend db methods for setting values to 'default' book
+flatdb.hack.set = function(db, key, value)
+	db.default[key] = value
+end
+
+-- extend db methods for watching new key
+flatdb.hack.guard = function(db, f)
+	setmetatable(db.default, {__newindex = f})
+end
+
+-- get key-value data from 'default' book
+print(db:get("hello"))
+
+-- set key-value data to 'default' book
+db:set("hello", "world")
+
+-- get key-value data from 'default' book
+print(db:get("hello"))
+
+-- set guard function
+db:guard(function(book, key, value)
+	print("CREATE KEY permission denied!")
+end)
+
+-- try creating new key-value pair to 'default' book
+db:set("key1", 1)
+db:set("key2", 2)
+
+-- update an existing key-value pair
+db:set("hello", "bye")
+
+print(db:get("key1")) -- prints nil
+print(db:get("key2")) -- prints nil
+print(db:get("hello")) -- prints 'bye'
+
+-- store 'default' book to './db/default' file
+db:save()
+
+```
+
+API
+==========
+
+- **Functions**
+
+  - **flatdb(dir) --> db**
+
+      Bind a directory as a database, returns nil if 'dir' doesn't exists. Otherwise, it returns a 'db' obeject.
+
+  - **db:save([book])**
+
+      Save all books or the given book(if specified) contained in db to file. The 'book' argument is a string, the book's name.
+
+- **Tables**
+
+  - **flatdb.hack**
+
+      The 'hack' table contains db's methods. There is only one method 'save(db, book)' in it by default.
+      It is usually used to extend db methods.
+
+Dependencies
+=======
+
+- [pp](https://github.com/luapower/pp)
+- [lfs](http://keplerproject.github.io/luafilesystem/)
+
+All above libraries can be found in [LuaPower](https://luapower.com/).
+
+License
+=======
+
+FlatDB is distributed under the MIT license.