markov.cpp: Bug fix, comment fixes.
markov.cpp: Bug fix, comment fixes.

maybe_perform_replacement: Bug fix to correctly disallow some ABA
transition sequences:
cuboctahedron -> truncated octahedron -> cuboctahedron,
truncated octahedron -> cuboctahedron -> truncated octahedron,
truncated octahedron -> octahedron -> truncated octahedron.

bustercopley/polymorph,bustercopley/polymorph