== cascade (resolution loop) ==
start:
OGOGOGOG
GOGOGOGO
RGOGOGOG
BBBOGOGO
RGOGOGOG
ROGOGOGO
OGOGOGOG
GOGOGOGO
round 1: cleared 3 cells
RBRGOGOG
OGOOGOGO
GOGGOGOG
RGOOGOGO
RGOGOGOG
ROGOGOGO
OGOGOGOG
GOGOGOGO
round 2: cleared 3 cells
RBRGOGOG
PGOOGOGO
YOGGOGOG
RGOOGOGO
OGOGOGOG
GOGOGOGO
OGOGOGOG
GOGOGOGO
cascade depth 2
ok: cascade resolves to a stable board