reserve z,z1,z2 for Complex;
reserve r,x1,x2 for Real;
reserve p0,p,p1,p2,p3,q for Point of TOP-REAL 2;

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;
