diff --git a/README.md b/README.md
index b0a019f36..169fad71e 100644
--- a/README.md
+++ b/README.md
@@ -32,17 +32,19 @@ The Go+ programming language is designed for engineering, STEM education, and da
 
 For more details, see [Quick Start](doc/docs.md).
 
+
 ## Key Features of Go+
 
 * A static typed language.
 * The simplest engineering language that can be mastered by children (script-like style).
 * Performance: as fast as Go (Go+'s main backend compiles to human-readable Go).
 * Fully compatible with [Go](https://github.com/golang/go) and can mix Go/Go+ code in the same package (see [Go/Go+ hybrid programming](doc/docs.md#gogo-hybrid-programming)).
-* No DSL (Domain Specific Language) support, but Specific Domain Friendly (see [DSL vs. SDF](doc/dsl-vs-sdf.md)).
-* REPL: see [iGo+](https://github.com/goplus/igop).
+* No DSL (Domain Specific Language) support, but it's Specific Domain Friendly (see [DSL vs. SDF](doc/dsl-vs-sdf.md)).
+* Support Go code generation (main backend) and bytecode backend (REPL: see [iGo+](https://github.com/goplus/igop)).
 * [Simplest way to interaction with C](doc/docs.md#calling-c-from-go) (cgo is supported but not recommended).
 * [Powerful built-in data processing capabilities](doc/docs.md#data-processing).
 
+
 ## How to install
 
 ### from source code
@@ -67,6 +69,7 @@ all.bat
 brew install goplus
 ```
 
+
 ## Go+ Applications
 
 ### 2D Games powered by Go+
diff --git a/doc/dsl-vs-sdf.md b/doc/dsl-vs-sdf.md
index 6167cc3e7..f539a4fd9 100644
--- a/doc/dsl-vs-sdf.md
+++ b/doc/dsl-vs-sdf.md
@@ -3,4 +3,173 @@ Go+ ClassFile: DSL vs. SDF
 
 Go+ doesn't support DSL (Domain Specific Language), but it's SDF (Specific Domain Friendly).
 
+The Go+ philosophy of Domain is:
+
+```
+Don't define a language for specific domain.
+Abstract domain knowledge for it.
+```
+
+Go+ introduces `ClassFile` to abstract domain knowledge.
+
+NOTE: `ClassFile` is still in beta and may be subject to change.
+
+What is `ClassFile`?
+
+TODO
+
+
+## DEMO: Go+ DevOps Tools
+
+This is a demo project to implement a new `ClassFile` work: [github.com/xushiwei/gsh](https://github.com/xushiwei/gsh).
+
+Before we dive into implementation of `github.com/xushiwei/gsh`, let's take a look how to use it.
+
+First, let's create a new module named [gshexample](https://github.com/xushiwei/gshexample):
+
+```sh
+mkdir gshexample
+cd gshexample
+gop mod init gshexample
+```
+
+Second, get `github.com/xushiwei/gsh` into `gshexample`:
+
+```sh
+gop get github.com/xushiwei/gsh
+```
+
+Third, create a Go+ source file named `./example.gsh` and write the following code:
+
+```coffee
+mkdir "testgsh"
+```
+
+Fourth, run this project:
+
+```sh
+gop mod tidy
+gop run .
+```
+
+It's strange to you that the file extension of Go+ source is not `.gop` but `.gsh`.
+
+But the Go+ compiler can recognize `.gsh` files as Go+ source because `github.com/xushiwei/gsh` register `.gsh` as its `ClassFile` file extension.
+
+We can change `./example.gsh` more complicated:
+
+```coffee
+type file struct {
+	name  string
+	fsize int
+}
+
+mkdir! "testgsh"
+
+mkdir "testgsh2"
+lastErr!
+
+mkdir "testgsh3"
+if lastErr != nil {
+	panic lastErr
+}
+
+capout => { ls }
+println output.fields
+
+capout => { ls "-l" }
+files := [file{flds[8], flds[4].int!} for e <- output.split("\n") if flds := e.fields; flds.len > 2]
+println files
+
+rmdir "testgsh", "testgsh2", "testgsh3"
+```
+
+### Check last error
+
+If we want to ensure `mkdir` successfully, there are three ways:
+
+The simplest way is:
+
+```coffee
+mkdir! "testsh"  # will panic if mkdir failed
+```
+
+The second way is:
+
+```coffee
+mkdir "testsh"
+lastErr!
+```
+
+Yes, `github.com/xushiwei/gsh` provides `lastErr` to check last error.
+
+The third way is:
+
+```coffee
+mkdir "testsh"
+if lastErr != nil {
+    panic lastErr
+}
+```
+
+This is the most familiar way to Go developers.
+
+### Capture output of commands
+
+And, `github.com/xushiwei/gsh` provides a way to capture output of commands:
+
+```coffee
+capout => {
+    ...
+}
+```
+
+Similar to `lastErr`, the captured output result is saved to `output`.
+
+For an example:
+
+```coffee
+capout => { ls "-l" }
+println output
+```
+
+Here is a possible output:
+
+```s
+total 72
+-rw-r--r--  1 xushiwei  staff  11357 Jun 19 00:20 LICENSE
+-rw-r--r--  1 xushiwei  staff    127 Jun 19 10:00 README.md
+-rw-r--r--  1 xushiwei  staff    365 Jun 19 00:25 example.gsh
+-rw-r--r--  1 xushiwei  staff    126 Jun 19 09:33 go.mod
+-rw-r--r--  1 xushiwei  staff    165 Jun 19 09:33 go.sum
+-rw-r--r--  1 xushiwei  staff    110 Jun 19 09:33 gop.mod
+-rw-r--r--  1 xushiwei  staff   1938 Jun 19 10:00 gop_autogen.go
+```
+
+We can use [Go+ powerful built-in data processing capabilities](doc/docs.md#data-processing) to process captured `output`:
+
+```coffee
+type file struct {
+	name  string
+	fsize int
+}
+
+files := [file{flds[8], flds[4].int!} for e <- output.split("\n") if flds := e.fields; flds.len > 2]
+```
+
+In this example, we split `output` by `"\n"`, and for each entry `e`, split it by spaces (`e.fields`) and save into `flds`. Condition `flds.len > 2` is to remove special line of output:
+
+```s
+total 72
+```
+
+At last, pick file name and size of all selected entries and save into `files`.
+
+
+### Implementation of gsh
+
+To most developers, they just use `ClassFile`. They don't need to implement a `ClassFile` project.
+
+We provide a `ClassFile` template project: [github.com/goplus/classfile-project-template](https://github.com/goplus/classfile-project-template).
+
 TODO