checkInvariants for both pre and post conditions

How about a modifier like this ?

```
modifier checkInvariants() {
if (fundBalance != this.balance) {
throw;
}
_
if (fundBalance != this.balance) {
throw;
}
}
```

Start by locating the contract that defines fundBalance and balance, then review how modifiers and invariant checks are currently handled. Clarify the intended precondition and postcondition behavior, including failure handling, before choosing an implementation. Done means the agreed modifier behavior is implemented and covered by tests.

Written by the indexing model from the issue text.