sub NRreciprocal {
    my( $n, $g ) = @_;

    my $x0 = $g;
    while( 1 ) {
        my $x1 = $x0 + $x0 * ( 1 - $n * $x0 );
        last if  ( $x0 - $x1 ) == 0.0;
        $x0 = $x1;
    }
    return $x0;
}