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 st p<>0.TOP-REAL 2 holds (Arg(p)<PI implies Arg(-p)=Arg(p)+PI)&
  (Arg(p)>=PI implies Arg(-p)=Arg(p)-PI)
proof
  let p;
  assume p<>0.TOP-REAL 2;
  then
A1: euc2cpx(p)<>0c by Th2,Th16;
  Arg(-p)=Arg(-euc2cpx(p)) by Th13;
  hence thesis by A1,COMPLEX2:13;
end;
