Adding a case to an existing enum with a protocol

Question

I want to create a protocol that enforces a certain case on all enums conforming to this protocol.

For example, if I have a <

Answer 1

An extension can add a nested enum, like so:

enum Plants {
  enum Fruit {
     case banana
  }
} 


extension Plants {
  enum Vegetables {
     case potato
  }
}