my @tocheck;
for my $value (values %$self) {
  push @tocheck, $value if ref($value) eq 'ARRAY';
}

my %seen;
for my $array (@tocheck) {
  $seen{$_}++ for @$array;
}

return grep { $seen{$_} == @tocheck } keys %seen;

##</code><code>##

my ($n, %seen);
for my $array (values %$self) {
  next unless ref($array) eq 'ARRAY'; # we're only interested in arrays
  $seen{$_}++ for @$array;
  $n++;
}
return grep { $seen{$_} == $n } keys %seen;

##</code><code>##

my %seen;
for my $value ( map { ref($_) eq 'ARRAY' ? @$_ : () } values %$self ) {
  $seen{$value}++;
}
return grep { $seen{$_} > 1 } keys %seen;