@matrices = qw(user_group user_realm group_realm
        realm_template template_subsite realm_subsite);

##</code><code>##

sub IsMatrix
{
        shift if UNIVERSAL::isa($_[0], __PACKAGE__);
        my ($child, $parent) = @_;
        my $cache = undef if undef;                                                                                                                              

        unless ($cache)
        {
                my %cache;
                @cache{@matrices} = (1) x scalar(@matrices);
                $cache = \%cache;
        }                                                                                                                              

        return $cache->{"${child}_$parent"};
}

##</code><code>##

grep /^${child}_$parent$/, @matrices;