## Workflow

`old_token_contract` is ERC20 compliant.
`new_token_contract` is `TokenSwap`

#### Token swap contract

```
import "zeppelin-solidity/contracts/token/ERC20/StandardToken.sol";

contract TokenSwap is StandardToken {
  event Swap(address indexed to, uint256 amount);

  address public old_token_contract;

  function TokenSwap(address _old) public {
    old_token_contract = _old;
  }

  function swapTokens(uint256 amount) public {
    // require: balanceOf msg.sender in old_token_contract is at least "amount"
    // require: allowance of new_token_contract to spend "amount" tokens of msg.sender
    // require: burn old tokens (transferFrom new_token_contract to 0x0)
    // mint new_token_contract "amount" tokens to msg.sender
  }

  function mintTokens(address _to, uint256 _amount) internal {
    // add "amount" to totalSupply_ of this token
    // add "amount" to balances[_to]
  }
}
```

1. `new_token_contract` inherits `TokenSwap` contract

2. Users `approve` the `new_token_contract` to spend their `old_token_contract` tokens

    `approve(new_token_contract_address, amount)`

3. Users call `swapTokens` in the `TokenSwap.sol` contract, with the amount they want to swap

    `TokenSwap.swapTokens(amount)`