function decreaseApproval(address _spender, uint _subtractedValue)
      public
      returns (bool)
    {
      uint oldValue = allowed[msg.sender][_spender];
      if (_subtractedValue > oldValue) {
        allowed[msg.sender][_spender] = 0;
      } else {
        allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
      }
      Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
      return true;
    }