Added more detailed use of the +list+ command on debugging guide

railties/guides/source/debugging_rails_applications.textile

@@ -328,6 +328,41 @@ If you repeat the +list+ command, this time using just +l+, the next ten lines o
 
 And so on until the end of the current file. When the end of file is reached, the +list+ command will start again from the beginning of the file and continue again up to the end, treating the file as a circular buffer.
 
+On the other hand, to see the previous ten lines you should type +list-+ (or +l-+)
+
+<shell>
+(rdb:7) l-
+[1, 10] in /PathToProject/posts_controller.rb
+   1  class PostsController < ApplicationController
+   2    # GET /posts
+   3    # GET /posts.xml
+   4    def index
+   5      debugger
+   6      @posts = Post.all
+   7
+   8      respond_to do |format|
+   9        format.html # index.html.erb
+   10        format.xml  { render :xml => @posts }
+</shell>
+
+This way you can move inside the file, being able to see the code above and over the line you added the +debugger+.
+Finally, to see where you are in the code again you can type +list=+
+
+<shell>
+(rdb:7) list=
+[1, 10] in /PathToProject/posts_controller.rb
+   1  class PostsController < ApplicationController
+   2    # GET /posts
+   3    # GET /posts.xml
+   4    def index
+   5      debugger
+=> 6      @posts = Post.all
+   7
+   8      respond_to do |format|
+   9        format.html # index.html.erb
+   10        format.xml  { render :xml => @posts }
+</shell>
+
 h4. The Context
 
 When you start debugging your application, you will be placed in different contexts as you go through the different parts of the stack.