Subdivide the grid into orthogonally connected regions such that regions of the same size do not share a border. The value of each cell in the grid is equal to its region size. Each "?" represents a digit from 1 to 9 which must be deduced. Dutch flat mates: every cell with value 5 either has a value-1 cell above it, or a value-9 cell below it (or both). Mod-3 groups: all cells with values which are equal modulo 3 form a single orthogonally connected group. This means every cell in the grid belongs to one of three orthogonally connected groups. (I recommend shading cells to keep track of groups.) For answer-check, replace 2-digit numbers with "0", and check that no digits have been rendered above clues.