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  my %set = ();
  my @new_items = ...;
  @set{@new_items} = (1) x @new_items;

  delete @set{@other_items};

  # Difference.
  sub difference {
...
    @symm{@set1, @set2} = (1) x (@set1 + @set2);
    \%symm;
  }