contract Erc20TokensContract {
    function transfer(address _to, uint256 _value);
    // returns (bool success); // not in CoinOffering Corporation.sol
    function balanceOf(address acc) returns (uint);
}
// 'interface':
//  this is expected from another contract,
//  where tokens (ERC20) are managed
function balanceOf(address acc) returns (uint);

// returns (bool success); // not in CoinOffering Corporation.sol