Note that they're using the message-passing idea: after an iterator has been created, you can call it with 'status?, 'dead?, 'alive?, or 'kill! to get/set it's private data.
What I'm suggesting is to treat all compound data types like that. You can then create function wrappers around them, which is in fact what they did too: note the definition of `iterator-empty?`
Every individual iterator has it's own private state, so it doesn't make sense to require extending built-in functions every single time you create a new iterator. Instead, you define a function wrapper (like `iterator-empty?`) that accesses the internal data for you. No need to extend anything! That conciseness is what message passing gives you.
