A non-empty subset A of X = X1 × · · · × Xd is a (proper) box if A = A1 × · · · × Ad and Ai ⊂ Xi for each i. Suppose that for each pair of boxes A, B and each i, one can only know which of the three states takes place: Ai = Bi, Ai = Xi \ Bi, Ai 6∈ {Bi, Xi \ Bi}. Let F and G be two systems of disjoint boxes. Can one decide whether ⋃ F = ⋃ G ? In general, the answer is ‘no’, but as is shown in th...