class TExec: public TNamed

   TExec is a utility class that can be used to execute a CINT command
   when some event happens in a pad.
   The command in turn can invoke a CINT macro to paint graphics objects
   at positions depending on the histogram or graph contents.

 Case 1:

 The TExec object is in the list of pad primitives (after exec.Draw()).
 When the pad is drawn, the TExec::Paint function is called. This function
 will execute the specified command.
 The following example uses the services of the class Aclock created
 in $ROOTSYS/test/Aclock.cxx.
 This examples uses a TTimer to redraw a pad at regular intervals (clock).
 When the clock is updated, a string with the current date&time is drawn.
{
   gSystem->Load("$ROOTSYS/test/Aclock");
   Aclock ck(400);
   gPad->SetFillColor(5);
   TDatime dt;
   TText t(.5,.3,"t");
   t.SetTextAlign(22);
   t.SetTextSize(.07);
   t.SetTextColor(4);
   t.Draw();
   TExec ex("ex","dt.Set();t.SetTitle(dt.AsString())");
   ex.Draw();
}

 Case 2:

 The TExec object may be added to the list of functions of a TH1 or TGraph
 object via hist->GetListOfFunctions()->Add(exec).
 When the histogram (or graph) is drawn, the TExec will be executed.
 If the histogram is made persistent on a file, the TExec object
 is also saved with the histogram. When redrawing the histogram in a
 new session, the TExec will be executed.
 Example:
     Assume an histogram TH1F *h already filled.
     TExec *ex1 = new TExec("ex1","DoSomething()");
     TExec *ex2 = new TExec("ex2",".x macro.C");
     h->GetListOfFunctions()->Add(ex1);
     h->GetListOfFunctions()->Add(ex2);
     h->Draw();
  When the Paint function for the histogram will be called, the "DoSomething"
  function will be called (interpreted or compiled) and also the macro.C.

 Case 3:

 A TExec object is automatically generated when invoking TPad::AddExec.
 Each pad contains a TList of TExecs (0, 1 or more). When a mouse event
 (motion, click, etc) happens, the pad object executes sequentially
 this list of TExecs. In the code (interpreted or compiled) executed
 by the TExec referenced command, one can call the pad service functions
 such as TPad::GetEvent, TPad::GetEventX, TPad::GetEventY to find
 which type of event and the X,Y position of the mouse.
 By default, the list of TExecs is executed. This can be disabled
 via the canvas menu "Option".
 See $ROOTSYS/tutorials/hist/exec2.C for an example.
    Root > TFile f("hsimple.root");
    Root > hpxpy.Draw();
    Root > c1.AddExec("ex2",".x exec2.C");
    When moving the mouse in the canvas, a second canvas shows the
    projection along X of the bin corresponding to the Y position
    of the mouse. The resulting histogram is fitted with a gaussian.
    A "dynamic" line shows the current bin position in Y.
    This more elaborated example can be used as a starting point
    to develop more powerful interactive applications exploiting CINT
    as a development engine.

 The 3 options above can be combined.