\begin{lisp:documentation}{xgcd}{FUNCTION}{x y }
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).
\end{lisp:documentation}