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kotlinlang

```fun Person.email(): Email? {
    contract {
        returnsNotNull() implies (<mailto:this@email.email|this@email.email> is String )
    }
    return this.email?.let {
        Email(it)
    }
}```
contract says: `Error in contract description: only references to parameters are allowed in contract description`. which i suppose makes sense, i’m trying to prove something about an internal value of a instance not directly related to the params/return?