Function req

req<Req extends object>(): ReqMarker<Req>
Creates a typed requirement marker for defineModelMixin.
Type Parameters
- Req extends object
  Shape that must exist on the base model instance.
Returns ReqMarker<Req>
Example
```
const producerMixin = defineModelMixin(
  { produced: tProp(types.number, 0) },
  req<{ quantity: number }>()
)
```