From Philippe:

This patch implements support for the accessing the collection object in
TTreeFormula:

//     Accessing collection objects
//     ============================
//
//  TTree::Draw default's handling of collections is to assume that any
//  request on a collection pertain to it content.  For example, if fTracks
//  is a collection of Track objects, the following:
//      tree->Draw("event.fTracks.fPx");
//  will plot the value of fPx for each Track objects inside the collection.
//  Also
//     tree->Draw("event.fTracks.size()");
//  would plot the result of the member function Track::size() for each
//  Track object inside the collection.
//  To access information about the collection itself, TTree::Draw support
//  the '@' notation.  If a variable which points to a collection is
prefixed
//  or postfixed with '@', the next part of the expression will pertain to
//  the collection object.  For example:
//     tree->Draw("event.@fTracks.size()");
//  will plot the size of the collection refered to by fTracks (i.e the
number
//  of Track objects).


git-svn-id: http://root.cern.ch/svn/root/trunk@11309 27541ba8-7e3a-0410-8455-c3a389f83636