Avoiding unintended copying / Making copying stand out
======================================================

Use a "copy" keyword everwhere where there is a copy?

Should this be done at the type level, expression, or both?


    func f(copy Thing th) -> copy Thing
    {
        copy Thing other
        other = copy th
        return copy do_stuff(copy th)
    }

Problems:
- This performs a shallow copy! That might not be clear to the user!
    - Is it possible to force deep copying somehow?
      Is it possible to make it work across modules?
- Maybe too much visual noise?

Solutions?
- Use a stack keyword instead?
- Use it on types and parameter expressions only?

    func f(stack Thing th) -> stack Thing
    {
        stack Thing other
        other = th
        return do_stuff(stack th)
    }