code: def LAST(COND, N1, N2):
      
      N2 = 1 if N2 == 0 else N2
      assert N2 > 0
      assert N1 > N2
      return COND.iloc[-N1:-N2].all()
docs: 表达持续性
      从前N1日到前N2日一直满足COND条件
  
      Arguments:
          COND {[type]} -- [description]
          N1 {[type]} -- [description]
          N2 {[type]} -- [description]