Let B be the collection of cells. If m = Extract(B), then m ∈ B and there does not exist m' ∈ B such that m' ∗ m holds: m1 ∗ m2 if one of the following holds: (1) m1 has only solid characters and m2 has at least one non-solid character (2) m2 has the "-" character and m1 does not, and, (3) m1 and m2 have d1, d2 > 0 dot characters respectively and d1 <d2.