code: def subset_(self, *args):
          
          df = self._subset(*args)
          if df is None:
              self.err("Can not get subset of data")
              return
          return self._duplicate_(df)
docs: Returns a Dataswim instance with a subset data based in positions
          Select a subset of the main dataframe based on position:
          ex: ds.subset(0,10) or ds.subset(10) is equivalent: it starts
          at the first row if only one argument is provided