function doApprove(address _spender, uint256 _amount) internal returns (bool success) {
        require(transfersEnabled);
        if (isContract(controller)) {
            require(TokenController(controller).onApprove(msg.sender, _spender, _amount));
        }
        allowed[msg.sender][_spender] = _amount;
        emit Approval(msg.sender, _spender, _amount);
        return true;
    }