because this will prevent the black cells from being contiguous.
- Alternative phrasing: a polymino of black cells that can only expand into
a single grey cell will expand there.
+#. Sometimes extending an $N -1 island to a white cell in proximity to one of
+its cells will cause the remaining cells to form a 4*4 black region. As
+a result, this white cell must be black.
+#. Often when there's a situation like:
+We can see that the island should expand into the leftmost gray/unknown cell
+or else it will need to take a costly detour which will consume the rest of its