code: def subset(self, subtxns):
          
          result = Positions(subtxns)
          if hasattr(self, '_frame'):
              result._frame = self._frame.ix[subtxns.pids]
              # passing in array results in index name being removed for some reason???
              if result._frame.index.name != self._frame.index.name:
                  result._frame.index.name = self._frame.index.name
          return result
docs: Construct a new Positions object from the new Txns object (which is assumed to be a subset)
          of current Txns object