Fill the grid with 0, −1 and +1. Every row and every column holds exactly two of each and sums to zero: each line is one 0-sphere (two dots, two vectors, two edges). Shaded cells are given. Tap a cell to cycle it.
MODEL Dave's 0-sphere: 0,0,−1,−1,+1,+1, Σ = 0. LIT the puzzle is a constraint-satisfaction problem; the generator checks with a solver that the clues admit a solution. ← SaPpH{{i}}on
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