docs: def abcell (a, b, c):
      """Given the nontrivial parameters for evaluation a 2D Gaussian as a
      polynomial, compute the equivalent ellipse parameters (major, minor, pa)
  
      Inputs: (a, b, c), where z = exp (ax² + bxy + cy²)
  
      Returns:
  
      * mjr: major axis (sigma not FWHM) of the Gaussian
      * mnr: minor axis (sigma not FWHM) of the Gaussian
      * pa: position angle (from +x to +y) of the Gaussian, radians
  
      """
      from numpy import arctan2, sqrt
  
      bad = _abccheck (a, b, c)
      pa = 0.5 * arctan2 (b, a - c)
  
      t1 = np.sqrt ((a - c)**2 + b**2)
      t2 = -t1 - a - c
      bad |= (t2 <= 0)
      mjr = t2**-0.5
  
      t2 = t1 - a - c
      bad |= (t2 <= 0)
      mnr = t2**-0.5
  
      w = np.where (bad)
      mjr[w] = np.nan
      mnr[w] = np.nan
      pa[w] = np.nan
  
      return ellnorm (mjr, mnr, pa)