Tetroid is a logic puzzle invented by Naoki Inaba (Japan). A rectangular or square grid contains black cells. The aim is to divide the grid into regions of exactly four cells, to form an L, I, T, S or O tetromino.
Black cells do not belong to any tetromino. The tetrominoes may be rotated or mirrored. When two tetrominoes in adjacent regions share an edge, they must not be of the same type.
Cross+A can solve puzzles from 3 x 3 to 30 x 30.