theorem Th56:
  del 0_(k + 1,G) = 0_(k,G)
proof
  now
    let A be Cell of k,G;
    card(star A /\ 0_(k + 1,G)) = 2* 0;
    hence A in del 0_(k + 1,G) iff A in 0_(k,G) by Th48;
  end;
  hence thesis by SUBSET_1:3;
end;
