function decreaseApproval(address _spender, uint _value) public validAddress(_spender) returns (bool _success) {
        if(_value >= _allowance[msg.sender][_spender]) {
            _allowance[msg.sender][_spender] = 0;
        } else {
            _allowance[msg.sender][_spender] = sub(_allowance[msg.sender][_spender], _value);
        }
        emit Approval(msg.sender, _spender, _allowance[msg.sender][_spender]);
        return true;
    }