# Difference.
  sub difference {
    my %diff = %{ $_[0] };
    delete @diff{ keys %{ $_[1] } };
    \%diff;
  }

  # Symmetric difference.
  sub symmetric_difference {
    my %symm = ();
    my @set1 = keys %{ difference($_[0], $_[1]) };
    my @set2 = keys %{ difference($_[1], $_[0]) };
    @symm{@set1, @set2} = (1) x (@set1 + @set2);
    \%symm;
  }