theorem Th4:
  for p1,p2 st euc2cpx(p1)=euc2cpx(p2) holds p1=p2
proof
  let p1,p2;
  assume
A1: euc2cpx(p1)=euc2cpx(p2);
  p2 = cpx2euc(euc2cpx(p2)) by Th2;
  hence thesis by A1,Th2;
end;
