Beast

A position is described as a “beast” when it is particularly challenging to

• somebody (or some algorithm) trying to analyse it, e.g. to find the best line of play or to evaluate it as a combinatorial game or
• the (makers of) rules (in which case it is a rule(s) beast), in the sense that one or more rule sets imply results that players find dissatisfying or counter-intuitive.

But sometimes “beast” is just a slangy way of referring to any (moderately interesting) group or position.

A collection of beasts is often referred to as a “bestiary”.

Most rules beasts have been specially constructed, but some have occurred in real games; they are in any case rare, and so barely affect most players. In either case, they are sometimes used in rules disputes to demonstrate a point.

A nasty, mean spirited opponent can be described as a beast.

Further reading

Analysis beasts

• BQM 222 — A difficult pro game situation (Lin – Sakata, 22nd Honinbo game 4)
  
• More Infinitesimals — Some infinitesimals in chilled go with multiple live options.
• Hanezeki — A seki where both sides decline to capture one or more stones in atari, one of them perhaps a hane on the first line
• A pathological seki — An apparent seki where two black groups share four liberties each with both of two white groups, which arose as a test-case for an algorithm

Rules beasts (or suggested to be such)

• Repetition rules
  • Cycle — Positions that return (at various lengths) to their starting point
  • Rules Beast 1 — superko surprisingly forces a connection in a seki to avoid a capture (John Tromp)
    
    • Superko puzzle 2 — Another superko puzzle
  • Sending two, returning one — A cycle that transfers points in territory scoring
  • Pinwheel Ko — In which kos rotate around the board, indefinitely in the absence of a superko rule
  • Unremovable ko — A ko which one player can start whenever they like, as their opponent cannot remove it
    • Unremovable ko for both sides — A ko which neither player can remove
  • Triple ko — Infamously considered a draw in some rule sets without superko
  • Leather Scroll Problem 64 / Solution — Bill wonders if this old triple ko problem was intended as a rules beast
  • Eternal Life (chosei) — A particular cycle
  • Spight Rules/Example — A ko both sides want to win but neither wants to fill
• Capturing rules
  • 1-eye flaw — An uncapturable group with one eye
  • Moonshine Life — Referenced in a historical rules dispute: a group with a false eye in ko that cannot be captured because the owner has unlimited ko threats
  • Sections “Seki with one…” and “…two false eye(s)” of page Seki — Cases of seki scored differently in different rule sets
  • Seki without shared liberties — Test cases by Matti Siivola for the ‘no territory in seki rule’
  • Suicidal Tendencies — Analysis of an item from Harry Fearnley’s bestiary (link below) exploring consequences of allowing suicide
  • Three points without capturing (Torazu sanmoku) — Japanese rules specified the result without it having to be played out
    
  • Five points without capturing (Torazu gomoku) — Japanese rules specify the result without it having to be played out
    
• https://www.cs.cmu.edu/~wjh/go/rules/siivola.html — Four rules beasts by Matti Siivola
  • http://www.helsinki.fi/~msiivola/tilanne5.html — A fifth rules beast by Matti Siivola (also in article Matti Siivola as of 2019-01-30)

Miscellaneous bestiaries

• • http://www.harryfearnley.com/go/bestiary/ — Harry Fearnley’s collection of beasts
  • http://denisfeldmann.fr/bestiary.htm - Denis Feldmann's bestiary. To quote its introduction, it is a collection of "Strange situations, famous tesujis and other wonders". In English, but also available in French.