[Question] Hierarchical domains with (hierarchical) roles

**What's your scenario? What do you want to achieve?**

Pretty much a follow-up from https://github.com/casbin/casbin/issues/718#issuecomment-889785940

> So, do you think this problem is solvable by Casbin? I think this is pretty typical use case of hierarchical RBAC where you do not want to define global role hierarchy into each domain.
>
> General idea would be pretty similar to the original question:
>
> - global domain
> - - subdomain1
> - - - lowersubdomain1
> - - subdomain2
>
> 1. So if `alice` is an `admin` in `global domain`, she should be an `admin` in all domains below it (`subdomain1`, `lowersubdomain1`, `subdomain2`).
> 1. This should happen without any additional policy lines. `g, alice, admin, global domain` should be sufficient
> 2. Defining what permissions `admin` has in each domain is still required.
> 2. If `bob` is an `admin` in `subdomain1`, he should have `admin` access to `lowersubdomain1`
> 1. But no access to any other domain

**Your model:**

```ini
[request_definition]
r = sub, dom, obj, act

[policy_definition]
p = sub, dom, obj, act

[role_definition]
g = _, _, _
g2 = _, _

[policy_effect]
e = some(where (p.eft == allow))

[matchers]
m = g(r.sub, p.sub, r.dom) && g2(r.dom, p.dom) && regexMatch(r.obj, p.obj) && regexMatch(r.act, p.act)
```

**Your policy:**

```
p, admin, global_domain, data1, (read|write)
p, admin, sub_domain, data2, (read|write)

g, alice, admin, global_domain
g, bob, admin, sub_domain
g2, sub_domain, global_domain
```

**Your request(s):**

```
alice, global_domain, data1, read --> true, correct
alice, sub_domain, data2, read --> false, incorrect
bob, sub_domain, data2, read --> true, correct
```