code: def lag(expr, offset, default=None, sort=None, ascending=True):
      
      return _shift_op(expr, Lag, offset, default=default,
                       sort=sort, ascending=ascending)
docs: Get value in the row ``offset`` rows prior to the current row.
  
      :param offset: the offset value
      :param default: default value for the function, when there are no rows satisfying the offset
      :param expr: expression for calculation
      :param sort: name of the sort column
      :param ascending: whether to sort in ascending order
      :return: calculated column