sub import {
    my $pkg = shift;

    if ($pkg ne __PACKAGE__) {
        foreach my $meth ( qw(foo bar) ) {
            $pkg->can($meth) or croak("Class $pkg does not define method $meth");
        }
    }

    my $is_exp = $pkg->isa('Exporter');
    $Exporter::ExportLevel++ if $is_exp;
    $pkg->SUPER::import(@_);
    $Exporter::ExportLevel-- if $is_exp;
}