Shade in some cells black such that the black cells are all connected to
each other through their edges, and no 2x2 cell area within the grid
contains all black cells. Numbers in a cell indicate the sizes of the
orthogonally contiguous groups formed by black cells sharing a corner or
an edge with that cell. For example, "3" means that the cell shares a
corner or an edge with three black cells, and they form a single group,
whereas "1 2" means the cell shares a corner with three black cells
which form a group of two cells and a single separate cell. Cells with
numbers cannot be black.