code: def inv_std_norm_cdf(x):
      
      z = 2*x -1
      ln1z2 = np.log(1-z**2)
      a = 8*(np.pi -3)/(3*np.pi*(4-np.pi))
      b = 2/(np.pi * a) + ln1z2/2
      inv_erf = np.sign(z) * np.sqrt( np.sqrt(b**2 - ln1z2/a) - b )
      return np.sqrt(2) * inv_erf
docs: Inverse cumulative standard Gaussian distribution
      Based on Winitzki, S. (2008)