global

def +(derived_candidate_solution)
  self.class.new(
    unmatched_expected_indexes + derived_candidate_solution.unmatched_expected_indexes,
    unmatched_actual_indexes   + derived_candidate_solution.unmatched_actual_indexes,
    # Ignore the indeterminate indexes: by the time we get here,
    # we've dealt with all indeterminates.
    [], []
  )
end