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====== How to use chains in ROOT ======
===== Idea =====
A chain is sort of a container for trees. A chain can contain an arbitrary number of trees and can be used the same way as a tree, as the class ''chain'' inherits from the class ''tree''.
===== Adding Files =====
A simple example is the following:
<code>
TChain* myChain = new TChain("DecayTree");
myChain->Add("myRootFile.root");
</code>
Note the following:
  * You have to add **files**, not **trees** to the chain
  * All your files need to have the same structure (subdirectories, branches,...)
  * If your trees are in a subdirectory, you have to specify it in the constructor. For example: If your tree ''DecayTree'' is in the subdirectory ''mySubdirectory'' in the root file ''myRootFile.root'', call the constructor with:
<code>
TChain* myChain = new TChain("mySubdirectory/DecayTree");
</code>

===== Accessing the chain =====
All the trees in the chain can be accessed the same way as it is done with trees, for example:
<code>
myChain->Draw("B_M");
</code>
to make a histogram with the B-mass.

**Note**: The first time you access the chain in a ROOT session, it takes a little longer, as the file has to be checked (there is a way around this, but it hasn't worked
out yet...)

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