Given the following code:

```
module MyModule {
  export interface IMyInterface {}
  export interface IMyInterfaceA extends IMyInterface {}
  export interface IMyInterfaceB extends IMyInterface {}

  function(my: IMyInterface): void {
    if (my instanceof IMyInterfaceA) {
      // do something cool
    }
  }
}

```

I get the error "Can't find name IMyInterfaceA". What's the correct way to handle this situation?