code: def _csalbr(tau):
      # Previously 3 functions csalbr fintexp1, fintexp3
      a = [
          -0.57721566,
          0.99999193,
          -0.24991055,
          0.05519968,
          -0.00976004,
          0.00107857,
      ]
      # xx = a[0] + a[1] * tau + a[2] * tau**2 + a[3] * tau**3 + a[4] * tau**4 + a[5] * tau**5
      # xx = np.polyval(a[::-1], tau)
  
      # xx = a[0]
      # xftau = 1.0
      # for i in xrange(5):
      #     xftau = xftau*tau
      #     xx = xx + a[i] * xftau
      fintexp1 = np.polyval(a[::-1], tau) - np.log(tau)
      fintexp3 = (
          np.exp(-tau) * (1.0 - tau) + tau**2 * fintexp1
      ) / 2.0
  
      return (
          3.0 * tau - fintexp3 * (4.0 + 2.0 * tau) + 2.0 * np.exp(-tau)
      ) / (4.0 + 3.0 * tau)
  
imports: from __future__ import annotations
  import logging
  from typing import Optional, Type, Union
  import dask.array
  import numpy
  import xarray
  from satpy.dataset.dataid import WavelengthRange
label_desc: Use image node_id 8 for calling a global function with example usage: _csalbr(tau) and returns: unknown