code: def AddPmf(self, other):
          
          pmf = Pmf()
          for v1, p1 in self.Items():
              for v2, p2 in other.Items():
                  pmf.Incr(v1 + v2, p1 * p2)
          return pmf
docs: Computes the Pmf of the sum of values drawn from self and other.
  
          other: another Pmf
  
          returns: new Pmf