{
    my $usage;
    BEGIN { $usage= '$oldVal= $obj->method( $idx [, $newVal ] )' }

    sub method {
        my $self= shift(@_);
        croak "Missing \$idx; usage: $usage\n"   unless  @_;
        my $i= shift(@_);
        my $old= $self->{B}[$i];
        $self->{B}[$i]= shift(@_)   if  @_;
        croak "Too many args; usage: $usage\n"   if  @_;
        return $old;
    }
}