[jph]

Great pull request. Ruby makes it easy to duplicate data by calling `.dup` or `+`, and this does help with state isolation. But duplication is an expensive operation.

Ruby's standard libraries don't have much support for immutability, or deep cloning, or copy on write, or linking concatenation. There's no standard library way to ask for a snapshot of an object.

So in the early days for Ruby, an idiom was: if you're writing a method that takes a list, and you need to be sure your list doesn't change out from under you, then duplicate it, get it working, and if it becomes a bottleneck then optimize it.