Custom graph invariants

Is there a some canonical way to enforce graph invariants in FGL?

I have been using FGL to implement a generalization of graphs that allows for arbitrary arity and nesting of relationships. It looks like this:

```
type RSLT = Gr Expr Role
data Expr = Word String
| Template [String] -- e.g. "_ needs _ to _"
| Relationship -- e.g. "Gotham needs Batman to hurry"
data Role = TemplateRole | Member Int
```

Words and Templates emit no edges. Each Relationship emits an edge labeled "TemplateRole" to a Template of arity k, and k more edges, labeled "Member i" for i in [1,k]. Thus the model permits lots of invalid state.

I don't know whether it's relevant, but I just [learned](https://www.reddit.com/r/haskell/comments/6tl5fp/invalid_state_complex_traversals_ways_to/) about the [dependent map](https://hackage.haskell.org/package/dependent-map) library, which implements a map that allows keys to specify the types of values associated with them.