New to Swift and saw code from Standford open course of Swift programming as below:

```
if pending != nil {
    acccumulator = pending!.binaryFunction(pending!.firstOperand,acccumulator)
    pending = nil
}

```

As I learn about unwrapping, `pending`, in this case, is ensured not to be nil inside `if` block, then why should I use `!` to unwrap pending while using it?