class TRef: public TObject

 TRef

 Persistent Reference link to a TObject
 A TRef is a lightweight object pointing to any TObject.
 This object can be used instead of normal C++ pointers in case
  - the referenced object R and the pointer P are not written to the same file
  - P is read before R
  - R and P are written to different Tree branches

 When a top level object (eg Event *event) is a tree/graph of many objects,
 the normal ROOT Streaming mechanism ensures that only one copy of each object
 in the tree/graph is written to the output buffer to avoid circular
 dependencies.
 However if the object event is split into several files or into several
 branches of one or more Trees, normal C++ pointers cannot be used because
 each I/O operation will write the referenced objects.
 When a TRef is used to point to a TObject *robj, for example in a class with
     TRef  fRef;
 one can do:
     fRef = robj;  //to set the pointer
 This TRef and robj can be written with two different I/O calls
 in the same or different files, in the same or different branches of a Tree.

 If the TRef is read and the referenced object has not yet been read,
 the TRef will return a null pointer. As soon as the referenced object
 will be read, the TRef will point to it. If the referenced object is
 contained in a TTree it can be autoloaded using the TBranchRef mechanism,
 which is set up by simply calling TTree::BranchRef().

 TRef also supports the complex situation where a TFile is updated
 multiple times on the same machine or a different machine.

 How does it work

 A TRef is itself a TObject with an additional transient pointer fPID.
 When the statement fRef = robj is executed, the following actions happen:
   - The pointer fPID is set to the current TProcessID.
   - The current ObjectNumber (see below) is incremented by one.
   - robj::fUniqueID is set to ObjectNumber.
   - In the fPID object, the element fObjects[ObjectNumber] is set to robj
   - ref::fUniqueID is also set to ObjectNumber.
 After having set fRef, one can immediatly return the value of robj
 using fRef.GetObject(). This function returns directly fObjects[fUniqueID]
 from the fPID object.

 When the TRef is written, the process id number pidf of fPID is written
 in addition to the TObject part of TRef (fBits,fUniqueID).
 When the TRef is read, its pointer fPID is set to the value
 stored in the TObjArray of TFile::fProcessIDs (fProcessIDs[pidf]).
 The pidf is stored as a UShort_t limiting a file to 65535 distinct
 ProcessID objects.
 The pidf is stored in the bits 24->31 of the fUniqueID of the TRef.
 This implies that the number of TRefs in a single ProcessID should not
 exceed 2**24 = 16777216.   For pidf greater than 254, the value 0xff is
 stored in those bits and we use the table TProcessID::fgObjPIDs which
 links the referenced object's address to its ProcessID.
 See section "ObjectNumber" below for a recipee to minimize the object count.
 If the objectnumber exceeds this limit, it could be the sign that:
   -The object count is never reset (see below)
   -TRef is misused.

 When a referenced object robj is written, TObject::Streamer writes
 in addition to the standard (fBits,fUniqueID) the pidf.
 When this robj is read by TObject::Streamer, the pidf is read.
 At this point, robj is entered into the table of objects of the TProcessID
 corresponding to pidf.

 WARNING1: If MyClass is the class of the referenced object, The TObject
          part of MyClass must be Streamed. One should not
          call MyClass::Class()->IgnoreTObjectStreamer()


 WARNING2: A TRef cannot point to another TRef.


 ObjectNumber

 When an object is referenced (see TRef assignement operator or TRefArray::Add)
 a unique identifier is computed and stored in both the fUniqueID of the
 referenced and referencing object. This uniqueID is computed by incrementing
 by one the static global in TProcessID::fgNumber. fUniqueID is some sort of
 serial object number in the current session. One can retrieve at any time
 the current value of fgNumber by calling the static function TProcessID::GetObjectCount
 or set this number via TProcessID::SetObjectCount.
 To avoid a growing table of fObjects in TProcessID, in case, for example,
 one processes many events in a loop, it might be necessary to reset the
 ObjectNumber at the end of processing of one event. See an example
 in $ROOTSYS/test/Event.cxx (look at function Build).
 The value of ObjectNumber (say saveNumber=TProcessID::GetObjectCount()) may be
 saved at the beginning of one event and reset to this original value
 at the end of the event via TProcessID::SetObjectCount(saveNumber). These
 actions may be stacked.

 Action on Demand

 The normal behaviour of a TRef has been described above. In addition,
 TRef supports also "Actions on Demand". It may happen that the object
 referenced is not yet in memory, on a separate file or not yet computed.
 In this case TRef is able to automatically execute an action:
   - call to a compiled function (static function of member function)
   - call to an interpreted function
   - execution of a CINT script

 How to select this option?
 In the definition of the TRef data member in the original class, do:
   TRef  fRef;   //EXEC:execName. points to something
 When the special keyword "EXEC:" is found in the comment field of the member,
 the next string is assumed to be the name of a TExec object.
 When a file is connected, the dictionary of the classes on the file
 is read in memory (see TFile::ReadStreamerInfo). When the TStreamerElement
 object is read, a TExec object is automatically created with the name
 specified after the keywork "EXEC:" in case a TExec with a same name does
 not already exist.
 The action to be executed via this TExec can be specified with:
    - a call to the TExec constructor, if the constructor is called before
      opening the file.
    - a call to TExec::SetAction at any time.
      One can compute a pointer to an existing TExec with a name with:
        TExec *myExec = gROOT->GetExec(execName);
      myExec->SetAction(actionCommand); where
      - actionCommand is a string containing a CINT instruction. Examples:
          myExec->SetAction("LoadHits()");
          myExec->SetAction(".x script.C");

 When a TRef is dereferenced via TRef::GetObject, its TExec will be
 automatically executed. In the function/script being executed, one or more
 of the following actions can be executed:
  - load a file containing the referenced object. This function typically
    looks in the file catalog (GRID).
  - compute a pointer to the referenced object and communicate this pointer
    back to the calling function TRef::GetObject via:
      TRef::SetStaticObject(object).
    When the TExec is called, it has access to the dereferencing TRef
    by calling GetStaticObject() (TRef::GetObject() sets fgObject to "this"
    before the call to TExec). This can be useful for accessing the TRef's
    fUniqueID.
 As soon as an object is returned to GetObject, the fUniqueID of the TRef is set
 to the fUniqueID of the referenced object. At the next call to GetObject,
 the pointer stored in fPid:fObjects[fUniqueID] will be returned directly.

 An example of action on demand is shown in $ROOTSYS/test/Event.h with
 the member:
      TRef    fWebHistogram;   //EXEC:GetWebHistogram
 When calling fWebHistogram.GetObject(), the function GetObject
 will automatically invoke a script GetWebHistogram.C via the interpreter.
 An example of a GetWebHistogram.C script is shown below
    void GetWebHistogram() {
       TFile *f= TFile::Open("http://root.cern.ch/files/pippa.root");
       f->cd("DM/CJ");
       TH1 *h6 = (TH1*)gDirectory->Get("h6");
       h6->SetDirectory(0);
       delete f;
       TRef::SetStaticObject(h6);
    }
 In the above example, a call to fWebHistogram.GetObject() executes the
 script with the function GetWebHistogram. This script connects a file
 with histograms: pippa.root on the ROOT Web site and returns the object h6
 to TRef::GetObject.
 Note that if the definition of the TRef fWebHistogram had been:
      TRef    fWebHistogram;   //EXEC:GetWebHistogram()
 then, the compiled or interpreted function GetWebHistogram() would have
 been called instead of the CINT script GetWebHistogram.C

 Special case of a TRef pointing to an object with a TUUID

 If the referenced object has a TUUID, its bit kHasUUID has been set.
 This case is detected by the TRef assignement operator.
 (For example, TFile and TDirectory have a TUUID)
 The TRef fPID points directly to the single object TProcessUUID (deriving
 from TProcessID) and managing the list of TUUIDs for a process.
 The TRef kHasUUID bit is set and its fUniqueID is set to the fUniqueID
 of the referenced object.
 When the TRef is streamed to a buffer, the corresponding TUUID is also
 streamed with the TRef. When a TRef is read from a buffer, the corresponding
 TUUID is also read and entered into the global list of TUUIDs (if not
 already there). The TRef fUniqueID is set to the UUIDNumber.
 see TProcessUUID for more details.

 Array of TRef

 The special class TRefArray should be used to store multiple references.
 A TRefArray has one single pointer fPID for all objects in the array.
 It has a dynamic compact table of fUniqueIDs. Use a TRefArray rather
 then a collection of TRefs if all TRefs stem from the same process.

 Example:
 Suppose a TObjArray *mytracks containing a list of Track objects
 Suppose a TRefArray *pions containing pointers to the pion tracks in mytracks.
 This list is created with statements like: pions->Add(track);
 Suppose a TRefArray *muons containing pointers to the muon tracks in mytracks.
 The 3 arrays mytracks,pions and muons may be written separately.