docs: def ellabc (mjr, mnr, pa):
      """Given a 2D Gaussian expressed as an ellipse (major, minor, pa), compute the
      nontrivial parameters for its evaluation.
  
      * 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
  
      Returns: (a, b, c), where z = exp (ax² + bxy + cy²)
  
      """
      _ellcheck (mjr, mnr, pa)
  
      cpa, spa = np.cos (pa), np.sin (pa)
      mjrm2, mnrm2 = mjr**-2, mnr**-2
  
      a = -0.5 * (cpa**2 * mjrm2 + spa**2 * mnrm2)
      c = -0.5 * (spa**2 * mjrm2 + cpa**2 * mnrm2)
      b = cpa * spa * (mnrm2 - mjrm2)
  
      return _abccheck (a, b, c)