@kha I have added support for opaque constants to the old C++ frontend,
and made sure the new frontend can still parse `library/init/core.lean`.
The kernel should enforce that opaque constants are really opaque, and
the following example should fail
```
constant x : nat := 0
theorem foo : x = 0 := rfl
```
If it doesn't, it is a bug.
Here are some remaining issues:
1- `environment.mk_empty` is currently an axiom because we cannot create
an inhabitant of an opaque type. A possible solution is to use
`option environment` instead of `environment`.
2- There is no support for opaque constants in the new
frontend. However, I modified it to handle axioms, and fixed the literal
values with decl_cmd_kind. I tried to mark some of my changes with
comments, but it is probably much easier for you to just check the
commit change list.
3- I did not add any support for automatically constructing `e`
at `constant x : t := e`. I think we can do this later
after we replace the old frontend with the new one. BTW, it took only a
few minutes to provide the inhabitants manually.
This suggestion has been discussed at Slack.
We have decided to use #"c" as notation because we wanted to allow `'`
in the beginning of identifiers like in SML and F*. In particular,
we wanted to allow users to use 'a 'b 'c for naming type parameters
like in SML. However, nobody used this notation. In the Lean standard
library, we are using greek letters for naming type parameters.
So, there is no real motivation for the ugly #"c" syntax.
