theorem Th15:
  euc2cpx(p1-p2)=euc2cpx(p1)-euc2cpx(p2)
proof
  thus euc2cpx(p1-p2)=euc2cpx(p1)+euc2cpx(-p2) by Th9
    .=euc2cpx(p1)+-euc2cpx(p2) by Th13
    .=euc2cpx(p1)-euc2cpx(p2);
end;
