TChain* AliRsnPrepareChain 
(
    const char *kCollectionFile="wn.xml",            // XML file containing tags
    Long64_t    nentries=TChain::kBigNumber
)
{
    
    // load rsn classes
    gROOT->LoadMacro("setupPar.C");
    bool unzip = kTRUE;
    setupPar("STEERBase",unzip);
    setupPar("ESD",unzip);
    setupPar("AOD",unzip);
    setupPar("ANALYSIS",unzip);
    setupPar("ANALYSISalice",unzip);
    setupPar("PWG2resonances",unzip);

    // connect to grid
    TGrid::Connect("alien://");
    
    TChain* analysisChain = new TChain("aodTree");

    // create chain of files to read
    TAlienCollection *myCollection = TAlienCollection::Open(kCollectionFile);
    if (!myCollection) {
        Error("AliRsnReadTaskRL", Form("Cannot create an AliEn collection from %s", kCollectionFile));
        return;
    }
    myCollection->Reset();
    // loop on the entries of the XML input file
    while ( myCollection->Next() ) {
        char esdFile[255];
        sprintf(esdFile, "%s", myCollection->GetTURL(""));
        Info("AliRsnReadTask", Form("Adding %s", esdFile));
        analysisChain->Add(esdFile);
    }

    Info("AliRsnReadTask", Form("CHAIN HAS %d ENTRIES", (Int_t)analysisChain->GetEntries()));
 
    return analysisChain;   
}