Define private class *methods* with Google Apps Script

Question

I want to create a class that has private methods. For example:

class RabbitDefiner {
  #define(){
    return "a rodent";
  }
}

I get:

Syntax error: ParseError: Unexpected token ILLEGAL

In other threads I've seen that private fields aren't implemented in GAS (why??) and there are some workarounds. I'm not sure how to apply those workarounds to methods. Has anyone found a solution to this?