Sometimes you add a new protocol (or interface) and you really want some existing types to implement that protocol. Some languages make that impossible (or real

Sometimes you add a new protocol (or interface) and you really want some existing types to implement that protocol. Some languages make that impossible (or really really ugly) when you didn't define those existing types yourself. What if the + operator didn't exist in Java? Think about how you would add it. Now think about how would like to be able to add it. What language features would have to be added to Java to support that?