StLorentzVector : Overloading
Example : invariant mass with 4-Vectors
StLorentzVectorD& electron(p1,p1.massHypothesis(electronMass));
StLorentzVectorD& positron(p1,p1.massHypothesis(electronMass));
double invmass = abs(electron + positron);
// abs(StLorentzVector&), operator+, can also do:
StLorentzVectorD& pair = electron + positron;
//operator=(StLorentzVectorD&),
double invmass = abs(pair); // or double invmass = pair.m();