[m-users.] How to declare a set of typeclasses?

having read 4.2.2 Equivalence types, I see that it is possible to create an equivalence type using type abbreviations but I can't figure out how to get one to work for a parameterised type class using sets.

:- type l1_set(T) == set(hittable(T)).

gives error:
	level_ufo.m:804: In definition of type `l1_set'/1:
	level_ufo.m:804:   error: undefined type `hittable'/1.

which is clear; hittable is indeed not a type, it's a typeclass, this was a deliberate 'process of elimination' for my thought process!

Then I did what I thought would work:

:- type l1_set(T) == set(hittable(T)) <= hittable(T).

but this gives the error:
	level_ufo.m:804: On the left hand side of type definition: error: type
	level_ufo.m:804:   parameters must be variables, but
	level_ufo.m:804:   `(l1_set(T) == set(hittable(T)))' and `hittable(T)' are not.

And here is the actual code where I am trying to use it, I am operating on the assumption that by default Mercury has some way of creating a unique hash for each object in the set to know what insert_new should do the right thing. I read "4.5. The Standard ordering" but I can't remember where I am sure I've seen about overriding this behaviour, it mentioned Haskell I think. Ah.. it is section 3.8 of the online page https://mercury-in.space/crash.html#orga13c54a

What am I not doing correctly? Is it possible?

Thanks,
Sean.

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