InputRange for data structure without order?

[Andy Valencia]

On Wednesday, 4 June 2025 at 02:11:18 UTC, H. S. Teoh wrote:
> The correct way to do this is to create a method in the 
> container that returns a range over its contents.  Popping the 
> range should NOT mutate the container.  It should be regarded 
> as something separate from the container.  Only then will you 
> get sane semantics.  Trying to conflate an unordered container 
> with a range will only lead to pain and bugs.

So, concretely, my container has an opApply method, and because 
of that foreach works to iterate its contents.  Imagine that I 
now want to chain() across several of these, along the lines of:

```d
     foreach(val; s1) {
         ...;
     }

     // But now:
     foreach(val; chain(s1, s2, s3)) {
         ...;
     }
```

What API do I add to my container so that chain will do its 
magic?  I don't see the infrastructure under std.range.chain 
using it.  Or is there a non-Range chain I haven't found yet?

Thank you again,
Andy