my $source = Sub::ParamObject->new({
    selector => [ qr/^(filename|url)$/, sub { @{$_[0]}{qw(filename url)} and !exists $self->{$_[1]} }, ],
    filename => [ qr/^\w+$/, sub { !exists $_[0]->{filename} or $_[0]->{selector} eq 'url' }, ],
    url => [ qr!^https?://!i, sub { !exists $_[0]->{filename} or $_[0]->{selector} eq 'url' }, ],
});