theorem Th7:
  cpx2euc(z1+z2)=cpx2euc(z1)+cpx2euc(z2)
proof
  cpx2euc(z1)+cpx2euc(z2) =|[Re z1 + Re z2, Im z1 + Im z2]| by EUCLID:56;
  hence thesis by Th6;
end;
