I'd like to define an interface with optional string values. Something like:

```
interface IEntity {
    values: ['RemainingUnits', 'ActualUnits', 'PlannedUnits']
}

```

But with this interface I have problems:

```
const entity0: IEntity = { values: ['PlannedUnits'] }; // => Error
const entity1: IEntity = { values: ['RemainingUnits', 'ActualUnits'] }; // => Error
const entity2: IEntity = { values: ['PlannedUnits', 'RemainingUnits', 'ActualUnits'] }; // => Error

```

So are there ways to write the correct interface to avoid errors above?

And perfectly without duplicated strings and not empty