reserve S for 1-sorted,
  i for Element of NAT,
  p for FinSequence,
  X for set;

theorem Th7:
  1.Z_2 + 1.Z_2 = 0.Z_2
proof
  1.Z_2 + 1.Z_2 = (1+1) mod 2 by Th6,GR_CY_1:def 4
    .= 0 by NAT_D:25;
  hence thesis;
end;
