EMPTY->BLUE: north = BLUE or (north = CYAN and south = WHITE)
EMPTY->CYAN: north = CYAN or east = BLUE or west = BLUE
EMPTY->WHITE: (south = BLUE and (not (north = NULL))) or south = WHITE

CYAN->EMPTY: south = EMPTY or south = CYAN
CYAN->BLUE: north = CYAN or north = BLUE or south = WHITE

BLUE->EMPTY: south = EMPTY
BLUE->CYAN: south = CYAN or west = EMPTY or east = EMPTY or north = WHITE

WHITE->EMPTY: not (north = NULL)
WHITE->CYAN: north = NULL