election_title: "Coombs Ex.21 — after: two twins join for B, and B's win becomes a coin flip"
scenario_description: |-
  The AFTER half of Felsenthal's Coombs twin example. Source: Dan S. Felsenthal (2010), Appendix A7, Example 21 — see coombs_ex21_twin_before.yaml for the setup.
  The same 20 voters plus two more casting B>A>D>C, twins of a voter already there: 22 in all. Their arrival adds two first places for B and, critically, two last places for C. C is now deleted first instead of A, the transfers run differently, and the count ends with A and B TIED — no winner without a lot.
  Before the twins arrived, B won outright with 11 of 20. Two extra ballots ranking B FIRST turned a certain win into a coin flip. That is the weak twin paradox: not that the twins made B lose outright, but that they made B's victory strictly less certain, which is already a violation of what any voter is entitled to assume about casting a ballot for their favorite.
  Because the post state is a genuine tie, this case has no determinate winner under Coombs and could never be published as a BetterVoting election with a meaningful result — the same limit noted for BV2142. The expected winner recorded below is the RCV-IRV winner, which IS determinate; only the Coombs count ties.
  Labels are Felsenthal's own A/B/C/D so the case can be read side by side with the paper's table.
  Tabulated as RCV-IRV, the mirror-image count; IRV elects B before and after, so the failure is Coombs' alone. For the Coombs count, and its tie warning, run tools_adam/pref_voting_tabulation_engine/coombs_report.py.
paradoxes: [twin]
voting_method: RCV_IRV
num_winners: 1
ballots: |-
  5:A>B>D>C
  5:B>C>D>A
  3:B>A>D>C
  6:C>A>D>B
  1:C>B>A>D
  2:C>B>D>A
expected_winners:
  - B

# file: coombs_ex21_twin_after.yaml