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+Tutorial
+========
+
+    WARNING: this documentation is a work-in-progress. Apologies.
+
+This tutorial shows you how to write extensions that are compatible with both
+`Scratch 2.0`_ and `Snap!`_.
+
+It assumes familiarity with at least one of these programming languages.
+Don't worry if you've only used one -- we'll explain the differences you need
+to know about.
+
+It also assumes familiarity with Python, and that you've already installed
+blockext. See install_ if not.
+
+Example
+-------
+
+Blockext is a Python module that makes writing extensions for these block-based
+programming languages much, much easier. It probably couldn't get any easier.
+Here's a quick example::
+
+    from blockext import *
+
+    light = False
+
+    @command("press light switch %n times")
+    def toggle_light(times=1):
+        global light
+        for i in range(times):
+            light = not light
+
+    @predicate("light is on?")
+    def is_light_on():
+        return light
+
+    menu("city", ["Barcelona", "Boston", "Brighton"])
+
+    @reporter("weather forecast for %m.city")
+    def forecast(city="Boston"):
+        import random
+        return random.choice(["windy", "snowy", "sunny"])
+
+    run("Tutorial Example", "example", 5000)
+
+Let's see it in action! Save and run the example, and then point your web
+browser to http://localhost:5000/. You'll then see a web page with the
+following options, above a list of the blocks you just defined:
+
+* Download Scratch 2.0 extension
+* Download Snap! blocks
+
+Click each of them to save the blocks to your downloads folder.
+
+In Scratch
+----------
+
+Now, load up the `Scratch 2.0 offline editor`_. Shift-click the "File" menu,
+and select "Import Experimental Extension". Select the ``scratch_example.s2e``
+file to load the extension blocks into Scratch.
+
+You can then select the purple "More Blocks" tab to see your blocks loaded into
+Scratch!
+
+Try "say"-ing the "light is on?" and "weather forecast" blocks, and try using
+the "press switch" block to change the value of the light reporter.
+
+In Snap!
+--------
+
+Open http://snap.berkeley.edu/run in your browser, and drag the
+``snap_example.xml`` file you just downloaded into the Snap! window.
+
+If that doesn't work, use "Import" from the "File" menu instead.
+
+Step-by-step
+------------
+
+Now you've seen the example extension in action, let's break down the code.
+Here's the first line::
+
+    from blockext import *
+
+This is just importing the entire contents of the ``blockext`` module. If
+you've done any Python, you should be familiar with this. Next::
+
+    light = False
+
+    @command("press light switch %n times")
+    def toggle_light(times=1):
+        global light
+        for i in range(times):
+            light = not light
+
+The ``@command`` line is called a decorator. You don't need to know how it
+works, just that a line starting with an ``@`` symbol always goes before a
+function, and does something special to the function.
+
+In this case, the ``command`` decorator is turning the function into a block
+definition for a command block. (Command blocks are the ones with the hole on
+the top and the puzzle-piece stub on the bottom.)
+
+The string just after the ``@command`` part is the text that will get used on
+the block
+
+The function's *name* is used internally so that Scratch/Snap! can recognise
+the block. This means that you can change the block's text without breaking
+existing projects that use your extension. As long as you don't change the
+function name, existing projects will still work.
+
+Let's have a look at the rest of the blocks::
+
+    @predicate("light is on?")
+    def is_light_on():
+        return light
+
+    @reporter("weather forecast for %m.city")
+    def forecast(city="Boston"):
+        import random
+        return random.choice(["windy", "snowy", "sunny"])
+
+I skipped over this line::
+
+    menu("city", ["Barcelona", "Boston", "Brighton"])
+
+This defines the options for the menu
+
+
+Now, the final line::
+
+    run("Tutorial Example", "example", 5000)
+
+
+* TODO: Doesn't crash if you throw an exception.
+
+  <pre class="b">
+    when green flag clicked
+  </pre>
+
+
+.. _Scratch 2.0: http://scratch.mit.edu/
+.. _Snap!: http://snap.berkeley.edu/
+.. _`Scratch 2.0 offline editor`: http://scratch.mit.edu/scratch2download/
+