theorem
  cpx2euc(z1-z2)=cpx2euc(z1)-cpx2euc(z2)
proof
  thus cpx2euc(z1-z2)=cpx2euc(z1+-z2) .=cpx2euc(z1)+cpx2euc(-z2) by Th7
    .=cpx2euc(z1)-cpx2euc(z2) by Th11;
end;
