class TTreeViewer: public TGMainFrame

 TreeViewer is a graphic user interface designed to handle ROOT trees and to
 take advantage of TTree class features.

 It uses ROOT native GUI widgets adapted for 'drag and drop' functionality.
 in the same session.
 The following capabilities are making the viewer a helpful tool for analysis:
  - several trees may be opened in the same session;
  - branches and leaves can be easily browsed or scanned;
  - fast drawing of branch expressions by double-clicking;
  - new variables/selections easy to compose with the built-in editor;
  - histograms can be composed by dragging leaves or user-defined expressions
  to X, Y and Z axis items;
  - the tree entries to be processed can be selected with a double slider;
  - selections can be defined and activated by dragging them to the 'Cut' item;
  - all expressions can be aliased and aliases can be used in composing others;
  - input/output event lists easy to handle;
  - menu with histogram drawing options;
  - user commands may be executed within the viewer and the current command
  can be echoed;
  - current 'Draw' event loop is reflected by a progress bar and may be
  interrupted by the user;
  - all widgets have self-explaining tool tips and/or context menus;
  - expressions/leaves can be dragged to a 'scan box' and scanned by
  double-clicking this item. The result can be redirected to an ASCII file;

 The layout has the following items:

  - a menu bar with entries : File, Edit, Run, Options and Help;
  - a toolbar in the upper part where you can issue user commands, change
  the drawing option and the histogram name, three check buttons Hist, Rec
  and Scan.HIST toggles histogram drawing mode, REC enables recording of the
  last command issued and SCAN enables redirecting of TTree::Scan command in
  an ASCII file (see -Scanning expressions-);
  - a button bar in the lower part with : buttons DRAW/STOP that issue histogram
  drawing and stop the current command respectively, two text widgets where
  input and output event lists can be specified, a message box and a RESET
  button on the right that clear edited expression content (see Editing...)
  - a tree-type list on the main left panel where you can select among trees or
  branches. The tree/branch will be detailed in the right panel.
  Mapped trees are provided with context menus, activated by right-clicking;
  - a view-type list on the right panel. The first column contain X, Y and
  Z expression items, an optional cut and ten optional editable expressions.
  Expressions and leaf-type items can be dragged or deleted. A right click on
  the list-box or item activates context menus.

 Opening a new tree and saving a session :

   To open a new tree in the viewer use <File/Open tree file> menu
 The content of the file (keys) will be listed. Use <SetTreeName> function
 from the context menu of the right panel, entering a tree name among those
 listed.
   To save the current session, use <File/Save> menu or the <SaveSource>
 function from the context menu of the right panel (to specify the name of the
 file - name.C)
   To open a previously saved session for the tree MyTree, first open MyTree
 in the browser, then use <File/Open session> menu.

 Dragging items:

 Items that can be dragged from the list in the right : expressions and
 leaves. Dragging an item and dropping to another will copy the content of first
 to the last (leaf->expression, expression->expression). Items far to the right
 side of the list can be easily dragged to the left (where expressions are
 placed) by dragging them to the left at least 10 pixels.

 Editing expressions

   Any editable expression from the right panel has two components : a
 true name (that will be used when TTree::Draw() commands are issued) and an
 alias. The visible name is the alias. Aliases of user defined expressions have
 a leading ~ and may be used in new expressions. Expressions containing boolean
 operators have a specific icon and may be dragged to the active cut (scissors
 item) position.
    The expression editor can be activated by double-clicking empty expression,
 using <EditExpression> from the selected expression context menu or using
 <Edit/Expression> menu.
    The editor will pop-up in the left part, but it can be moved.
 The editor usage is the following :
   - you can write C expressions made of leaf names by hand or you can insert
   any item from the right panel by clicking on it (recommandable);
   - you can click on other expressions/leaves to paste them in the editor;
   - you should write the item alias by hand since it not only make the expression
  meaningfull, but it also highly improve the layout for big expressions
   - you may redefine an old alias - the other expressions depending on it will
   be modified accordingly. An alias must not be the leading string of other aliases.
  When Draw commands are issued, the name of the corresponding histogram axes
  will become the aliases of the expressions.

 User commands can be issued directly from the textbox labeled "Command"
 from the upper-left toolbar by typing and pressing Enter at the end.
   Another way is from the right panel context menu : ExecuteCommand.
 All commands can be interrupted at any time by pressing the STOP button
 from the bottom-left
 You can toggle recording of the current command in the history file by
 checking the Rec button from the top-right

 Context menus

   You can activate context menus by right-clicking on items or inside the
 right panel.
 Context menus for mapped items from the left tree-type list :
   The items from the left that are provided with context menus are tree and
 branch items. You can directly activate the *MENU* marked methods of TTree
 from this menu.
 Context menu for the right panel :
   A general context menu is acivated if the user right-clicks the right panel.
   Commands are :
   - EmptyAll        : clears the content of all expressions;
   - ExecuteCommand  : execute a ROOT command;
   - MakeSelector    : equivalent of TTree::MakeSelector();
   - NewExpression   : add an expression item in the right panel;
   - Process         : equivalent of TTree::Process();
   - SaveSource      : save the current session as a C++ macro;
   - SetScanFileName : define a name for the file where TTree::Scan command
   is redirected when the <Scan> button is checked;
   - SetTreeName     : open a new tree whith this name in the viewer;
   A specific context menu is activated if expressions/leaves are right-clicked.
   Commands are :
   - Draw            : draw a histogram for this item;
   - EditExpression  : pops-up the expression editor;
   - Empty           : empty the name and alias of this item;
   - RemoveItem      : removes clicked item from the list;
   - Scan            : scan this expression;
   - SetExpression   : edit name and alias for this item by hand;

 Starting the viewer

   1) From the TBrowser :
  Select a tree in the TBrowser, then call the StartViewer() method from its
 context menu (right-click on the tree).
   2) From the command line :
  Start a ROOT session in the directory where you have your tree.
 You will need first to load the library for TTreeViewer and optionally other
 libraries for user defined classes (you can do this later in the session) :
    root [0] gSystem->Load(\"TTreeViewer\");
 Supposing you have the tree MyTree in the file MyFile, you can do :
    root [1] TFile file(\"Myfile\");
    root [2] new TTreeViewer(\"Mytree\");
 or :
    root [2] TreeViewer *tv = new TTreeViewer();
    root [3] tv->SetTreeName(\"Mytree\");