class AliRsnPairDef: public TObject

  This class is a simple set of definitions which are used to define a
  decay tree to be studied for a resonance, in terms of the PID and charge
  of its candidate daughters, which in turn determins what kind of objects
  the analysis must take into account.
  This object contains two AliRsnDaughterDef which define a model for each
  of the two expected daughters (see also AliRsnDaughterDef class) plus a
  mass hypothesis for the resonance, which is used for computin quantities
  which need it (like rapidity or Mt), and a PDG code, which is used to
  check for true pairs, when needed. In all other cases, these two additional
  values can be left to their default (meaningless) value.
  Since this object must define a decay channel, the only provided constructor
  allow to set a PID and a charge.

  authors: A. Pulvirenti (alberto.pulvirenti@ct.infn.it)
           M. Vala (martin.vala@cern.ch)