code: def index(self, col: str):
          
          df = self._index(col)
          if df is None:
              self.err("Can not create index")
              return
          self.df = df
docs: Set an index to the main dataframe
  
          :param col: column name where to index from
          :type col: str
  
          :example: ``ds.index("mycol")``