;;; XGCD (x y)                                                       [FUNCTION]
;;;    Extended gcd; the call
;;;         (xgcd X Y)
;;;    returns a multiple value list:
;;;    - GCD
;;;    - U,V  such that they solve the equation
;;;        GCD=U*X+V*Y
;;;    - U1,V1 such that
;;;        LCM=U1*X=V1*Y (up to the sign).
;;;