code: def log_sum_exp_ab(a, b):
      
      if a > b:
          return a + np.log(1. + np.exp(b - a))
      else:
          return b + np.log(1. + np.exp(a - b))
docs: log_sum_exp for two scalars.
  
      Parameters
      ----------
      a, b: float 
  
      Returns
      -------
      c: float
          c = log(e^a + e^b)