code: def AExn(mt, x, n):
      
      return (mt.Mx[x] - mt.Mx[x + n]) / mt.Dx[x] + mt.Dx[x + n] / mt.Dx[x]
docs: AExn : Returns the EPV of a endowment insurance. 
      An endowment insurance provides a combination of a term insurance and a pure endowment