sub set_comm  # named after the unix utility "comm"
{
  my @ar = @_; # two sets, passed as array(ref)s.
  # make the corresponding sets:
  my @hr = map { my %h; @h{@$_} = (); \%h } @ar;

  # figure out which is shorter, since we'll be iterating
  # over that list of keys:
  my( $shorter, $longer ) =
    keys %{$h[0]} < keys %{$h[1]}
    ? ( 0, 1 )
    : ( 1, 0 );

  # determine common keys:
  my @common =
    grep { exists $hr[$longer]{$_} }
    keys %{$hr[$shorter]};

  # remove those from the sets:
  delete @{$hr[0]}{@common};
  delete @{$hr[1]}{@common};

  # return "in first only, in second only, in both":
  return( [ keys %{$hr[0]} ], [ keys %{$hr[1]} ], \@common );
}