fixed requirements

docs/concept/concept.rst

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+Concepts
+========
+
+The principles are :
+
+* Short code
+* Automatic widget placement
+* Introspection
+* Same class names
+* Coherent style
+
+We are going to create the **tklib** library which redefines the following objects 
+from the **tk** module and the newer themed  **ttk** module:
+
+* Frame
+* Label
+* Button
+* Checkbox
+* Radiobutton
+* Entry
+* Combobox
+* Listbox
+* Scrollbar
+* SizeGrip
+* Text
+* Progressbar
+* Scale
+* Spinbox
+* Canvas
+
+
+The module ``tklib`` is a wrapper around ``tkinter``. 
+It's role is to simplify the placement of widgets and the creation of applications.
+
+When we import ``tklib`` we import automatically ``tk`` and ``tkk``::
+
+    from tklib import *
+    print(dir())
+
+This is what we get when we print ``dir()``::
+
+    ['App', 'Button', 'Callback', 'Canvas', 'Checkbox', 'Combobox', 
+    'ContextMenu', 'Entry', 'Frame', 'Image', 'ImageGrab', 'ImageTk', 
+    'Inspector', 'Item', 'Label', 'Labelframe', 'Listbox', 'ListboxSearch', 
+    'Menu', 'Notebook', 'Panedwindow', 'Radiobutton', 'Scale', 'Scrollable', 
+    'Scrollbars', 'Separator', 'Spinbox', 'Text', 'Treeview', 'Window',
+    '__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', 
+    '__loader__', '__name__', '__package__', '__spec__', 'math', 'os', 're', 
+    'sys', 'tk', 'ttk']
+
+Let's create a root object::
+
+    root = tk.Tk()
+
+Comparision of ``tk``, ``ttk``, ``tklib`` Buttons
+-------------------------------------------------
+
+Now we create a classic (tk) button, a themed (ttk) button and a 
+button from the ``tklib`` module. 
+All ``tklib`` widgets already have convenient arguments. They automatically
+
+* get their master from a stack
+* have a default label (the widget name)
+* have a default placement method (grid)
+
+This code creates three buttons using different constructors::
+
+    tk.Button(root, text='tk.Button').grid()
+    ttk.Button(root, text='ttk.Button').grid()
+    Button()
+
+    root.mainloop()
+
+This is how the three buttons look. We see a little bit of grey background 
+around the newer themed buttons.
+
+.. image:: concept1.png
+
+Default appearance of ``tklib`` widgets
+---------------------------------------
+
+Let's add some more widgets and look at the result. 
+The ``tk.Tk()`` root window does not have a themed background color. It remains white.
+The theme's background color only is adopted after inserting the first frame.
+After inserting a frame, the frame becomes the new master and all subsequent
+widgets are embedded. Frames can be nested::
+
+    root = tk.Tk()
+
+    Button()
+    Label()
+    Entry()
+    Scale()
+
+    Frame()
+    Spinbox()
+    Combobox()
+    Checkbox()
+    Radiobutton()
+
+    root.mainloop()
+
+Here is the result. We notice the default labels Button, Label, Check and Radio.
+
+.. image:: concept2.png
+
+
+Input widgets with labels, default value and callbacks
+------------------------------------------------------
+
+Usually an input widget needs a label next to it, so the user knows its meaning.
+We therefore add a ``label`` option to the widget. If the ``label`` option is set,
+a label is placed in front of the input widget and a frame is placed around them.
+
+When an input widget is modified, a callback function can be called::
+
+    def cb(event=None):
+        print('callback')
+
+Sometimes it is convenient to declare an executable command string, instead of 
+having to declare a separate callback function. This command string is executed 
+within the widget environment and ``self`` refers to the widget itself. Here 
+is an example of the two types of callback functions. Both have the label *Combobox*, 
+both have the values *Mon;Tue;We*. The first one calls the callback function ``cab``;
+the second one executes the command string ``print(self.get())`` and prints its value::
+
+    Combobox('Combobox', 'Mon;Tue;Wed', cb)
+    Combobox('Combobox', 'Mon;Tue;Wed', 'print(self.get())', 'Wed')
+
+It is also convenient to have default values for the input widgets. 
+We add a ``val`` option which allows to set an inital value::
+
+    Entry('Entry', cb)
+    Entry('Entry', 'print(self.get())', 'text')
+
+    Scale('Scale', cb)
+    Scale('Scale', 'print(self.get())', 0.5)
+
+    Spinbox('Spinbox', cb)
+    Spinbox('Spinbox', 'print(self.get())', 50)
+
+This is the screen capture of the above exemple:
+
+.. image:: concept3.png
+
+Automatic placement
+-------------------
+
+Widgets are placed automatically with the ``grid`` method.
+
+.. autoclass:: concept4.Demo
+
+.. image:: concept4.png