code: def resolve_rpn(self, rpndict):
          
          if rpndict == {}:
              return {}
  
          retflag = self.get_rpndict_flag(rpndict)
          cnt = 0
          tmpdict = {k: v for k, v in rpndict.items()}
          while not retflag:
              # update rpndict
              cnt += 1
              tmpdict = self.update_rpndict(tmpdict)
              # and flag
              retflag = self.get_rpndict_flag(tmpdict)
          return tmpdict
docs: solve dict of rpn expressions to pure var to val dict
          :param rpndict: dict of rpn expressions
          return pure var to val dict