my %hit;
foreach my $t (@term) {
  my @doc = $documents_mentioning{$t};
  $hit{$_}++ for (@doc);
}
# Filter (keys %hits) by # of hits (at least 2), then sort.
my @res = sort { $hit{$b} <=> $hit{$a} }
  (grep {$hit{$_} >= 2} keys %hit);