
  You want to solve this nonlinear system of equations :

        a X**2 + b Y**2 + c Z**2 = d
        e X**2 + f Y**2 + g Z**2 = h
        i X**2 + j Y**2 + k Z**2 = l

  If you write :

               X**2 = x
               Y**2 = y
               Z**2 = z
    
  You can rewrite the system :

        a x + b y + c z = d
        e x + f y + g z = h
        i x + j y + k z = l

 Which is a linear system of equation in x, y, z.

 Suppose, the solution of the system is :

                    x = r
                    y = s
                    z = t 

 you can write
                 X**2 = r    if r =  9    X = -3   or X = 3
                 Y**2 = s    if s = 25/4  Y = -5/2 or X = 5/2
                 Z**2 = t    ...
 
 The bad values for r, s, t are for example negative numbers
