reserve a,b,c,d,e,f for Real,
        g           for positive Real,
        x,y         for Complex,
        S,T         for Element of REAL 2,
        u,v,w       for Element of TOP-REAL 3;

theorem Th07:
  0 <= a^2
  proof
    per cases;
    suppose a = 0;
      hence thesis;
    end;
    suppose a <> 0;
      hence thesis by SQUARE_1:12;
    end;
  end;
