@union = union(\@a, \@b);
@inter = intersection(\@a, \@b, \@c);
@diff  = difference(\@a, \@b);
@in_a  = unique_to(\@a, (\@b, \@c));


sub union {
  my %seen;
  @seen{@$_} = () for @_;
  return keys %seen;
}


sub intersection {
  my %seen;
  for (@_) { $seen{$_}++ for @$_ }
  return grep $seen{$_} == @_, keys %seen;
}


sub difference {
  my %seen;
  for (@_) { $seen{$_}++ for @$_ }
  return grep $seen{$_} == 1, keys %seen;
}


sub unique_to {
  my %seen;
  @seen{@{ shift() }} = ();
  delete @seen{@$_} for @_;
  return keys %seen;
}