docs: def abcd2 (x0, y0, a, b, c, x, y):
      """Given an 2D Gaussian expressed as the ABC polynomial coefficients, compute
      a "squared distance parameter" such that
  
         z = exp (-0.5 * d2)
  
      Inputs:
  
      * x0: position of Gaussian center on x axis
      * y0: position of Gaussian center on y axis
      * a: such that z = exp (ax² + bxy + cy²)
      * b: see above
      * c: see above
      * x: x coordinates of the locations for which to evaluate d2
      * y: y coordinates of the locations for which to evaluate d2
  
      Returns: d2, distance parameter defined as above.
  
      This is pretty trivial.
  
      """
      _abccheck (a, b, c)
      dx, dy = x - x0, y - y0
      return -2 * (a * dx**2 + b * dx * dy + c * dy**2)