sub method {
    my $self= shift(@_);
    croak 'Missing $idx; usage: $oldVal= $obj->method( $idx [, $newVal ] )',$/
      unless  @_;
    my $i= shift(@_);
    my $old= $self->{B}[$i];
    $self->{B}[$i]= shift(@_)   if  @_;
    croak 'Too many args; usage: $oldVal= $obj->method( $idx [, $newVal ] )',$/
      if  @_;
    return $old;
}