Failure to instantiate inductive type with identical type

Documenting the issue mentioned in #377.

The following code fails to unify the two `nat`s with error ``type `nat` incompatible type declaration``:

```
type nat = [O | S of nat].

theory Second.
  type nat = [O | S of nat].
end Second.

clone Second as Twost with
  type nat <- nat.
```

This also fails if the two types come from two different clones of the same theory.