I have a struct which I would like to have a public opIndex which
returns by value (so client code can't modify my internal array),
and a private version which allows the implementing code to
modify stuff with `this[whatever] = whatever`.
I tried to to write 2 versions of opIndex:
```
public Type opIndex(IndexType x) const {... }
//and
private Type ref opIndex(IndexType x) { ... }
```
which doesn't seem to work because client code that has a
non-const reference to my container tries to use the private
non-const version and triggers a `not accessible` error. Is
there a way to do this with overloads, or will I need to just
pick a different name for the private version?
