reserve a,b,c,x,y,z for Real;

theorem
  for x,y being Complex st x^2=y^2 holds x=y or x=-y
proof
  let x,y be Complex;
  assume x^2=y^2;
  then (x-y)*(x+y)=0;
  then x-y=0 or x+y=0;
  hence thesis;
end;
