code: def _adjust_returns(returns, adjustment_factor):
      
      if isinstance(adjustment_factor, (float, int)) and adjustment_factor == 0:
          return returns
      return returns - adjustment_factor
docs: Returns the returns series adjusted by adjustment_factor. Optimizes for the
      case of adjustment_factor being 0 by returning returns itself, not a copy!
  
      Parameters
      ----------
      returns : pd.Series or np.ndarray
      adjustment_factor : pd.Series or np.ndarray or float or int
  
      Returns
      -------
      adjusted_returns : array-like