I have a failure initializer that takes a string, if this string contains incorrect characters (T, A, C, G) I want to return nil:

I tried something like this, unsuccessful:

```
init?(strand: String) {
    let success = strand.contains(where:  { !"TACG".contains($0) })
    if !success {
        return nil
    }

    self.strand = strand
}

```

I somehow got confused by the two `contains` calls, so I am not sure if my check is correct.  

Any help is appreciated.