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
  for p holds Arg p in ].0,PI.[ iff p`2 > 0
proof
  let p;
  Im euc2cpx(p)=p`2 by COMPLEX1:12;
  hence thesis by COMPLEX2:18;
end;
