theorem Th59:
  for G being Grating of d9 + 1, C being Chain of (d9 + 1),G holds
  del C` = del C
proof
  let G be Grating of d9 + 1, C be Chain of (d9 + 1),G;
  thus del C` = del(C + Omega(G)) by Th55
    .= del C + del Omega(G) by Th58
    .= del C + 0_(d9,G) by Th57
    .= del C;
end;
